Has anyone thought of using 4 'donut' tires?

[Peakster]

I'm not sure if this has been proposed for an experiment in the past, but I'll ask anyways:

Has anyone tried replacing all 4 of their tires with the skinny "donut" tires that can be inflated to 75psi? Almost all donut tires are designed to last over 5,000km (3,100 miles) yet they are rarely used and can be found in junk yards for a song and dance.

Makes me think of how easy it is to pedal speed bikes with the skinny tires compared to the thick tires of mountain bikes. The thinner tires might improve aerodynamics too?

- Peakster

[rh77]

I'm not sure if this has been proposed for an experiment in the past, but I'll ask anyways:

Has anyone tried replacing all 4 of their tires with the skinny "donut" tires that can be inflated to 75psi? Almost all donut tires are designed to last over 5,000km (3,100 miles) yet they are rarely used and can be found in junk yards for a song and dance.

Makes me think of how easy it is to pedal speed bikes with the skinny tires compared to the thick tires of mountain bikes. The thinner tires might improve aerodynamics too?

- Peakster

That is a good idea, but from my experience with do-nut spares, the recommended distance is to last 50 miles and the speed is limited to 45 mph. I wore one out once because I had to work over a Holiday weekend and the tire stores were closed. The tread was nearly gone over about 125 miles and 55-60 mph.

I have wanted a lighter tire and less of a footprint to propel, but it just seems to be cost-ineffective right now.

RH77

[ketel0ne]

http://www.lucasclassictires.com/

Portawalls would work, you would need wheels though, could be a challenge.

[onegammyleg]

Changing to a significantly narrower tyre than what the car came with when new could encur fines from those blue guys and in some places ANY deviation from standard can get you a defect notice.

[zpiloto]

I think you would need to see how much it would increase the rpms for a given speed and see how that would affect the FE. The increase in RPM might cancel the other benifets.

[kickflipjr]

I think 2 donuts in the rear might help a bit and be safer.

plus it would be easier to make rear wheel skirts :)

[onegammyleg]

MMM -- donuts :D

[onegammyleg]

Hi theclencher -¨and one pair of smaller and really skinny tires¨

In many countires missmatched tyres like this would result in a defect notice and/or a fine.
It may be law , but perhaps the cops may not be eager (or observant) to chase it up.

I remember our local chineese resturaunt owner had a brand NEW Benz , and he had some after market (but very high quality) mags and tyres fitted.
Perhaps it was racially motivated , perhaps not , but a cop stopped him and defected his car , giving him 7 days to change back to original equipment or go unregistered.

[psyshack]

Okmulgee, Ok. is the only place in my life where Ive seen 4 donut tires on cars. You see this from the welfare crowd. its there last chance effort to stay ,oble around here. And it dosent last long. :)

[rh77]

Changing to a significantly narrower tyre than what the car came with when new could encur fines from those blue guys and in some places ANY deviation from standard can get you a defect notice.

I've seen cars that look like skateboards, Caprices with 22-inch chrome wheels, racing-only tires on the street, dark tint, etc. It seems like around here either you've committed a crime, look like a drug car, or are speeding to get busted. For state inspections (50-states, 50 different methods, some states no inspection at all) -- just take in the stock wheels (or here get a shop you know will pass you). The Police is generally pretty busy with other stuff.

Geez- I thought our cops were ******!

Police Officers are out there putting their lives on the line, so I see it a bit differently.

RH77

[Wazabi Owner]

About a month and a half ago I had a flat on my Insight and had to put the donut on. Mileage dropped SIGNIFICANTLY just from using one over my stock RE92's. So at least with a vehicle that is already running LRR tires it should drop the mileage :confused: . For something else who knows?

[elg]

I´ve done it in mpg purpose !

First test Saab 9000i.
(donut for Saab is 115/70-16, in stock 185/65-15)
Got 4 "donuts" from junkyard and tested to run down a hill. Ran about 5 times (let the car roll free and messured distance). Then I changed back to stock wheels I got in the trunk, just to make sure that the mass of the car should be an other. Ran 5 times again and in total there was a very small hint that the "donuts" were better but I desided that was just faith or something. Much more less than 1%.
BUT I started the downhill-test with the recomended 2.1 bar pressure in stock tire and there was a clear difference when I later filled them to 3.5 bar.
That was for real !
Second test with the Saab was out on the road. No Difference at all.
Third test was highspeed test, about the drag. I let the car slow down in neutral from 150km/h to 100km/h on a flat not windy road. No dif.

This was maybe two years ago but I belive also had them on for a week but no dif.

The Nissan I now driving has also been under the donut test, I´m not finished yet but after 2000km I blow one front tire so it may take a while till I put them on again.

At all testings, Speed and distance were compared to GPS. Donut pressure around 4 bar.

For my personal, narrow tires=less drag and less friction.

// Elg

[Ryland]

the other day I took my spare donut out of the trunk and inflated it, realized it weighed a ton, so I set it on the scale, 21 pounds, compared to 23 pounds of my vx rims and 165/70 tires (weight new) so I'm temped to replace my donut with another vx rim, and a narrow tire, something like a 155/80-13, as it would be narrower and lighter then a wider tire, hell, next time I get tires I might just get 155/80's all around, unless I can find some 145/80-13's like VW diesel rabbits came with.

[jolt-tsp]

Unless you never exceed 30mph, I wouldn't do this. If you need to brake hard, you're putting all the weight up front, spread across two tires that combined are maybe the width of one normal tire. Your 60-0 distances will be far increased. Also, swerving to avoid an accident could very well put you out of control.

[omgwtfbyobbq]

Heh, I made the same claim and got scolded by a physics grad. Ideally, braking ability, with solid tires, only depends on the
http://upload.wikimedia.org/math/e/f/8/ef81341333267ccfa0674812a23ae636.png
normal force and coefficient of friction between the surfaces.
As we start using two surfaces that can't be approximated as solid, like sticky drag racing tires on asphalt, or mountain bike tires on sand, then the area plays a significant part. But for donuts, the biggest risk is wear and inflation. Wear can be monitered, and inflation is just noting the speed rating/max pressure and never exceeding that. Braking will primarily be influenced by the difference in tire compound.

[elg]

Just to make sure no missunderstandings.
Donut I used were with the flat driving suface and made by michelin.

There are one other model, almost like a tire for motorcykle, I tried that one on the car but after inflated 5 bar the surface to the ground was the same almost, depressed by cars weight so I never tested these.

Maybe how ever the pattern of my Donut (The tested version) is very small and narrow one with a lot of friction surface to ground, the legal 155 has wide water canals. I have not messured that, other with my sight.

This is a guess but anyway, donut of 115 compared to a 145 or 155mm tire of low friction type, I don´t belive the difference is that big. In friction matters. Ofcause in aerodynamic even if it´s a small one.
BUT don´t go fast on Donuts and don´t go long, It´s just for temparary use !!! A few km before front tire blowed my 3year son was in the car.

So there is a lot to do for saving money and fuel but this trick is maybe the last one to do.
Here VW sold a model called Lupo 3L, diesel version, that one came with 145mm Michelins and these were special Low-friction. Thats what I´m going for instead of donuts.

// elg

[Peakster]

P.S. Peakster- what are you doing sitting on that hood- you'll crush it!

Also what is that thing in your avatar?


I was extra careful not to crush it, so there was no damage to the car when I took that picture.

As for the avatar, that's my 2005 RezMobile Miser that I made in my 1st year university. I have a webpage set up for the vehicle at http://www.cardomain.com/ride/567996 if you want to see it sometime.

[Ryland]

I was looking around on some tire web sites, and it looks like everyone but the US has narrow tires avalible to them, down to 135/85-13" it's crazy, why are amaricans obsessed with big cars, wide tires, and poor gas mileage?

[red91sit]

another thing to think about, 4 skinny tires at 40psi have the same size contact patch as a regular tire at 40psi, since they're all applying 40psi to the ground, it uses the same amount of area to support the weight of the vehicle.

[jolt-tsp]

It is good practice to be able to quantify your claims- otherwise they are just speculation. Logical, educated guesses- yes, but unproven nonetheless.

While that made you look smart, you are incorrect. It is not a guess. When racing you deal largely with altering your contact patch and dealing with the effects. I'm not going to come on here and argue the fundamentals of traction. I gave my two sense, if you choose to ignore it, that no blood on my hands. If someone would like to bring a logical counter-argument to my attention, I will be more than happy to indulge them with my side of the argument.

[onegammyleg]

Hi Ryland -¨why are amaricans obsessed with big cars, wide tires, and poor gas mileage?¨

Are they ?
,, Or are they just buying what they are allowed to get ? . perhaps the decision has already been made for them and that it is a sellers driven market not a buyers market as people believe.
Remember , the more gas you burn , the more tax money the government gets.
The more taxes they get the happier they are.

I dont think any government wants gas economy cars untill they have worked out how to tax the heebyjeebies out of a new fuel source and then they will push a move toward that and abandon gas.

Either way , we are well and trully .....
http://www.keiths.org/office-humor/screwed.jpg

[rh77]

Post-War 1950's brought a boom of suburb home building, and the consequent system of roads. The automobile was/is a symbol of independence. Over time, people liked the luxury of big cars, even if they could get one cheap.

LSS, tradition has kept a majority of buyers, from this generation, to buy big cars (large percentage of the population). Without mass transit, younger generations follow the same pattern and tradition still holds. Fact is, most Americans don't care about gas mileage until it hits them hard in the wallet. A survey shows that we're getting "Greener", but is it a phase, and what will it produce?

Luckily, Honda and Toyota changed some minds in the '80s onward to shift the paradigm to smaller, more fuel efficient vehicles. Then the SUV craze started (incorrect perception of safety, convenience, and dominance). Companies had to follow the trend to keep profits up.

It can be argued: but if more people demand smaller cars, do you think we'll get them??? I'm thinking so...

RH77

edited for grammar, punctuation

[Matt Timion]

Hi Ryland -¨why are amaricans obsessed with big cars, wide tires, and poor gas mileage?¨

Are they ?
,, Or are they just buying what they are allowed to get ? . perhaps the decision has already been made for them and that it is a sellers driven market not a buyers market as people believe.
Remember , the more gas you burn , the more tax money the government gets.
The more taxes they get the happier they are.

I dont think any government wants gas economy cars untill they have worked out how to tax the heebyjeebies out of a new fuel source and then they will push a move toward that and abandon gas.

Either way , we are well and trully .....


THis hits it on the head, and perhaps we're in need of a new thread as to not hijack this one.

I'm working on an article right now that will be syndicated about how most of Americas problems are due to the auto manufacturers not allowing us to have a better selection. Part of it I'm sure is due to the gov't safety regulations... which is another important article that must be written. Have fatalities gone DOWN as a result of stricter safety laws?

[diamondlarry]

The way I see it, ultimately it's the buyer that has the control, they make the purchase decision and fork over the money and they don't have a gun to their head. But the seller does everything in their power to influence the sale.

Buyers do have options; for example they could have been choosing crx's, sprints, metros, etc. for years and in vast numbers instead of F150s, Explorers, Suburbans, etc.... but they didn't. They could have but they didn't.

They opted to buy into the line the manufacturers were selling them- "safety", convenience, capability, status, all that. Never mind that it's largely a crock.

I noticed that during the 10 year (?) run of U.S. Metro sales, I never saw a single TV ad for them. Not one. Nothing in the newspapers either. I might have seen one in a mag, but it could have been a story about that ad and not the actual ad? HOWEVER, turn on the TV, open a newspaper, or read a mag and you will see a deluge of adverts for suvs and 4x4s.

I think the U.S. national fleet is composed of more gas hogs than necessary. (understatement of the year!) I blame the buyer for choosing to be selfish, gluttonous, and willfully ignorant. I blame the manufacturers for enabling that line of thought.
AMEN! I agree. Honda killed the Insight this year and I don't ever remember seeing an ad on TV, or anywhere for that matter, for the Insight.

[MetroMPG]

Trucks and SUVs make the automakers vastly more profit than sub-compact cars. It's no surprise which they are flogging harder for the sales.

[omgwtfbyobbq]

While that made you look smart, you are incorrect. It is not a guess. When racing you deal largely with altering your contact patch and dealing with the effects. I'm not going to come on here and argue the fundamentals of traction. I gave my two sense, if you choose to ignore it, that no blood on my hands. If someone would like to bring a logical counter-argument to my attention, I will be more than happy to indulge them with my side of the argument.

We're on the complete opposite end of the spectrum. Racing tires have a very soft compound, and leave a lot of themselves on the road so to speak. In terms of economy, everyone here, given the chance, is probably running the hardest tire available at above the sidewall pressure rating, because that's where we can minimize rolling friction.
It's two completely different situations. On one hand, racing requires traction for high speed cornering and braking, so a high wear/sticky tire is an advantage, and because it's sticky, the greater the contact patch, the greater it sticks to the pavement, the greater the traction. Otoh, economy requires the minimization of traction (and speed) in order to maximize how well the cars roll. So in this case we're effectively dealing with two solid surfaces, and the difference in traction between a donut and a LRR tire is very small compared (heck, the donut might have better traction) to the LRR tire and the racing tire.

[jolt-tsp]

"I'm not going to come on here and argue the fundamentals of traction"- uh, isn't that exactly what you're doing?

No, that is what you're doing. I just came in and stated quick facts. The only thing I came back arguing was your assumption that I was making guesses.

What the issue may be, as omgwtfbyobbq pointed out was the opposite ideas of how we want our tires to perform. The a skinny tire with less traction will improve efficiency, there is no question about that. So if you're not at all concerned about safely, than my earlier comments can be disregarded, as I was speaking solely from a safety standpoint. I had read previously on this board where people speak against tailgating a semi for safety reasons, so I made the assumption that this was an issue of some importance for this board.

Your argument that a 5000 lbs Microbus stops on a dime with 100mm tires is not something I would consider a logical argument. So, as I mentioned, I will not argue that.

As far as the article you loosely described (that is also 20 years old) there are ways to use the traction of your other tires when one is dead.

-If the tire is on the inside of the car, it is not supporting hardly any weight during a skidpad test, so you would not expect a dramatic change in performance.
-On the outside rear, the LSD will put the torque to the inside where there is sufficient traction.
-On the outside front (this would be the hardest test to pass) the driver would have to use the torque of the vette (of which it has plenty) to keep the car from understeering.

The last test mentioned should show fair reduction in lateral g's, but should still be a street-able number. However, all that test shows us is that driving on one spare is not significantly more dangerous than driving on the four OE tires (on a RWD car with an LSD).

As I said, I was just offering information, I'm not trying to start an argument.

[onegammyleg]

¨with an LSD¨

Yeah , man , LSD , ime hearin ya .... KOOOL
http://www.dailyhaha.com/_pics/crazy_illusion.jpg

[jolt-tsp]

That Bus can throw you right out of your seat on those 4 puny contact patches. That is something I can easily prove (to anyone in the Bus anyway) and it is germane to the subject. That the vehicle can stop and corner well with small contact patches is the issue. Millions of motorists in Kei cars and other dinky-tired vehicles can attest to that.


To say the traction is sufficient for your taste is different from saying that the contact patch makes no difference in stopping power. Getting thrown from your seat is no impressive feat and I would more than expect four spares to at least have that much grip.

I'm not saying your car will slide forever when you hit the brake, I'm just saying that your stopping distance will increase when you decrease the CoF (which occurs when using more narrow tires).

I fully understand if you feel the stopping power to be safe. I was only stating I do not feel compact spare tires will yeild a short enough stopping distance over about 35 mph for my taste.

While you know millions are not having problems stopping with their thin tires, a friend of mine running 165's on a 2200lbs Sentra rear ended a truck primarily due to the tires. He was only travelling 30 mph and following about 2 seconds behind the truck. He's a cautious driver that has some racing experiance, so I have no reason to doubt his reaction time.

[jolt-tsp]

Ya, I would expect a decent set of 175s or 185s slightly overinflated would be the norm for most the cars on this board. I ran 185s on my Civic until I turned it track car. :thumbup:

No 315s for me, 255 is going to be the upper limit on my Civic's new wheels :(

[Lug_Nut]

And, back to Fuel Efficiency, wasn't that the reason for the original post?
Four temporary "donut" tires may be skinny, but they aren't low rolling resistance. Like the Swedish poster, I tried four temp tires, new, unused from a junkyard and found that rolling tests and coast down tests had the T135-15 at 60 psi (the oem spare size for my car) donuts stopping or slowing sooner than a set of Michelin 195-60-14 (oem size, but not the oem brand) inflated to 32 psi.

[Spule 4]

I think 2 donuts in the rear might help a bit and be safer.

plus it would be easier to make rear wheel skirts :)

The earlier Citroen ID/DS cars had smaller tires in the rear, along with the side skirts. 155R400 on the first and later were 165R380 (15).

http://www.channel4.com/4car/media/100-greatest/03-large/27-citroen-ds.jpg

These cars are also streamlined underneath, with holes for the motor and exhaust. Fun cars, but with the age of them now, ownership and daily use could become costly.

[molecule]

i think that is the prototype bed for ricky in the old tv show 'silver spoons'
(op's avatar)

:gigglesnort:

[onegammyleg]

The earlier Citroen ID/DS cars had smaller tires in the rear, along with the side skirts. 155R400 on the first and later were 165R380 (15).

Those French without doubt come up with some of the most ugliest cars in the world ,, but inside them they pack quite modern , if not too modern ideas.
The Citroen 2CV was an amzing little thing ..reputedly it could be driven over a plowed field without several hay bails ballanced on top of it falling off.
Very fine chasis dynamics.

My brother had a Renault 16 for a while ...

http://doidosporclassicos.com/galeria1/0298.jpg

... and for a small car the ride was just as smooth as a large car.
One would say ¨plush¨

I wouldnt say that I like french cars as such , but I do observe them with interest.

[Spule 4]

The Renault R16 was Renaults big car at the time. They are nice, and very advanced, the seating arrangement is the only car I have seen with more options than the Fit/Jazz from Honda.

A lot of people do not realize also that the French (Citroen) beat Mercedes with a production diesel car by three years.

[psyshack]

AMEN! I agree. Honda killed the Insight this year and I don't ever remember seeing an ad on TV, or anywhere for that matter, for the Insight.

Honda is world famous for doing something better than anybody else in the world to get a record or prove a point. Then let it slip quitely away. They have done this for year with there motorcycles.

They set out to prove a point with Insight. They did no futher development, and did not market the car. It was there plan to do it like that Im sure.

BAD HONDA, BAD HONDA!!!!

Heck they basicly blew it with the R18 in my car. They could have made it a lean burn and also put a Si type Vtec in it. But iin one swope they would have ruined there Hybrid and Si. LMAO

[Spule 4]

Honda is world famous for doing something better than anybody else in the world to get a record or prove a point. Then let it slip quitely away. They have done this for year with there motorcycles.

They set out to prove a point with Insight. They did no futher development, and did not market the car. It was there plan to do it like that Im sure.

BAD HONDA, BAD HONDA!!!!

Heck they basicly blew it with the R18 in my car. They could have made it a lean burn and also put a Si type Vtec in it. But iin one swope they would have ruined there Hybrid and Si. LMAO

They Hybrid is Dead, Long Live the Hybrid.

Honda is smart to be getting out while they can with the Insight and the Accord. With the changes in diesel rules and technology, the good Doktor's Oil Motor will be the new hot item in the next couple years. Even Honda are working on this technology as we speak.

[Ted Hart]

Just now, I am beginning to see GM TV commercials that make a point of "9 out of 10 of our vehicles get over 30 mpg". That's great, but WHERE HAVE THEY BEEN THE LAST 20 YEARS?!?

SUVs are all about profits! Why make cars when the same number of SUVs make more money? Move over, car! Make room for more money!:cool:

[landspeed]

I think the following video is probably a definitive comment on the use of 4 donut tyres on a car:

http://www.youtube.com/watch?v=T_1A4Mghz_4&mode=related&search=

It is FF drifting, and general craziness. Notice, just after one of the guys hanging out of the car punches the police car (37 seconds), they start doing
some FF drifting (47 seconds), You can see how thin the tyres are - they deliberately use those tyres so that they can slide around more easily. They can also drive completely sideways (e.g. at 55 seconds), without flipping the car over, despite doing very high speeds. It is also easy to lose control, and in fact they almost died at about 1 minute, by hitting a post!

[The Toecutter]

Honda complains it never sold enough Insights, but the real truth is, it never made enough to meet demand.

[Peakster]

Honda complains it never sold enough Insights, but the real truth is, it never made enough to meet demand.

This is a bit off topic, but I remember the very first ad I saw for the Honda Insight:

An old Volkswagen Bus comes to a stoplight and the camera shows all the "Tree-Hugger" statements plastered all over the bus. Next, a weird looking car (the Insight) pulls alongside it, and I think both vehicles drive away when the light turns green. Right after that, there's text stating the Insight name and the (then) ludicrously high fuel economy it got.

Ever since then, I've never seen another ad for the insight :( . Too bad. it was a good one.

[Ryland]

185mm wide tires are crazy, even 175's are to wide for my taste, I feel very confident cornering as fast as I want with my 165mm wide tires, altho they were hard to find, they were well worth the search, and I think my next set is going to be 155mm if I can find some in a high performance modle.

[red91sit]

I think the following video is probably a definitive comment on the use of 4 donut tyres on a car:

http://www.youtube.com/watch?v=T_1A4Mghz_4&mode=related&search=

It is FF drifting, and general craziness. Notice, just after one of the guys hanging out of the car punches the police car (37 seconds), they start doing
some FF drifting (47 seconds), You can see how thin the tyres are - they deliberately use those tyres so that they can slide around more easily. They can also drive completely sideways (e.g. at 55 seconds), without flipping the car over, despite doing very high speeds. It is also easy to lose control, and in fact they almost died at about 1 minute, by hitting a post!

Hmm, I'd have to disagree with you a little here. I think they are normally on stock tires, but at 47 seconds they do defintly look extra small/thin.

More imporantly, you can slide around just like they were doing with regular tires, and a removed front sway bar. For example, right after the first time I put a full tank of E-85 in the Lincoln, I was driivng home (on a closed course) on a divided highway. As I approached my corner I thought it would be fun to pitch it sidways (80mph) so I did. It did not roll, the body leaned until the suspension was completley resting on the bump stops (happens a lot), but there was no time I've ever been worried about rolling the car. It's nearly impossible unless you have an SUV or R-compound tires.

::EDIT:: hahaha, I just remebered the look on the drivers face of a pickup who was waiting at the corner! I'm sure he thought I almost died.

[Peakster]

Bought myself another never-used donut for my Geo for $8 a few days back. I could do a mini test sometime by attaching them to the front-drive wheels, but I want to wait until the snow clears up more.

Possible advantages to donut tires (for fuel economy):
1) less tire contact area
2) can be inflated to 60psi
3) lowers the vehicle
4) less rotational weight
5) faster acceleration

Impending disadvantages:
1) makes gearing shorter (bad for highway)
2) handling and braking characteristics worsen
3) low tread life
4) speedometer/odometer will show higher than actual speed/distance
5) makes wheel well space larger
6) might throw out alignment (but probably not)

[Peakster]

Now that the Geo is done being repaired at the shop, I decided to put the 2 donut tires on the front to give them a try and see how they feel:

Before:
175

After:
176

It did a good job of lowering the nose a bit but look at that massive wheel well space! I placed the bigger tires in the rear hatch, so my weight is the same. The odometer reads 248,547.35 kilometers when I did this, so I'll make a mental note that my speedometer will be optimistically high from this point on. This will also be needed to be taken into consideration since 1/2 of my tank was with the original tires, and the next half will be with the donuts. I plan to keep them on for as long as possible so I get a good sample. A-B-A testing really isn't practical since it still feels like 3*F outside.

Edit: I just went on the highway to see the difference in the speedometer. I drove 100km/h (read from the Scangauge II) and the "your speed" sign on the highway read 91km/h. I've noted the sign before with the normal tires and it's very accurate.

This means that all my speeds/distances will need to be multiplied by 0.91 to give the actual speed/distance.

[zpiloto]

Looks like you could run duct tape over those wheel wells and cover a considerable amount of the space. I even think that it comes in green.

[budomove]

185mm wide tires are crazy, even 175's are to wide for my taste, I feel very confident cornering as fast as I want with my 165mm wide tires

Agreed!

just drove 185/65/14s on hxs after driving on 165/70/13 vx's for a while, and whoa, i felt like i was glued to the ground. It is too much for 70hp. I love the 165/65/14 lrr potenzas on my new hx's, and am hoping the 175/65/14s I will be putting on soon won't be too much. They are the lrr kumho 795, so hopefully they will be smooth and quiet, and as light as kumho claims. 14lbs, we'll see.

[Peakster]

After a short 81.3 mile drive today with the donut tires this is what I have to report:

The Good:
1). Incredible coasting capability. With the donuts on, the Geo can CODFISH like I couldn't imagine!
2) Uphill climbing is a sinch! I drove up a steep hill (one that usually takes me from 60-45mph by the time I get to the top) and the car only decreased in speed by about 3-4 mph (from 55mph, real speed) by the time I got the the top. No foot pressure was needed, the cruise control had it down pat.
3) Braking seemed to be just as good as before.

The Bad:
1) Speed is shown to be higher (both on SG and speedometer) so when I'm doing 100km/h (62mph) I'm actually only doing about 91km/h (56mph). This means the odometer needs adjustment for figuring out FE.
2) the Geo rides like a tank! I have to go across a gravel road to get to my cheap gas station and the glovebox kept opening from all the nasty bumps.
3) the RPMs are higher (for the actual speed) than if I had the original tires on. 3000 RPM (@ 60mph, real speed with regular tires and wheels) divided by 0.91 is around 3300 RPM (@ 60mph real speed, with donut tires).

The Questionable:
1) The car's acceleration seems to be a lot faster whether in city or on highway, but again this could be an illusion as I'm technically going slower than what the SG and speedo says.
2) Handling. The car did have a considerable amount of understeer on icy turns, but remember: I had ice-trac tires on. So this comparison would be unfair.
3) Aero improvements/disadvantages. The car is a bit lower now and there is less width to the tires, but now there is more space in the wheel well.

And Finally: the appetizer FE results (19.4*F with 30mph N/W winds and cruise control on flat terrain):

Going north-west:
55mph: 41.0mpg (actually 37.3 mpg @ 50 mph)
50mph: 42.8mpg (actually 38.9 mpg @ 45.5 mph)
45mph: 45.5mpg (actually 41.7 mpg @ 41 mph)

Going south-east:
55mph: 58.8mpg (actually 53.5 mpg @ 50 mph)
50mph: 64.8mpg (actually 59.0 mpg @ 45.5 mph)
50mph: 65.5mpg (actually 59.6 mpg @ 41 mph)

I've even had a couple of 'Current MPG' readouts in the mid 70s @ ~45 mph real speed. Granted that's only high 60s in real FE, but that has never happened to me before on highway driving.

So in other words FE increase/decrease is not conclusive with this information. It's also really strange that FE (according to the SG) was pretty much the same @ 45.5 mph real speed compared to 41 mph real speed (this pattern was predominant thought the trips).

Overall, I'm really intrigued by the unusual results. I think it's definitely worth A-B-A testing when the weather gets nice in the future. Like I said in a previous post, I'm going to keep the 'donut' configuration on for as long as possible and see how my future tanks turn out (just filled my tank today and came out with 48+ mpg).

[Peakster]

Be careful those tires are not designed to last very long and have a max speed around 50 mph. Let us know how the tread wear goes.

Yeah that's what I've been told. I checked the wear just a few minutes ago and it seems fine, for now.

Actually, *looks at owner's manual*: "The compact spare is made to perform well at posted speed limits for distances up to 3,000 miles (5 000 km), so you can finish your trip and have your full-size repaired or replaced where you want."

[Lug_Nut]

The Good:

2) Uphill climbing is a cinch! I drove up a steep hill (one that usually takes me from 60-45mph by the time I get to the top) and the car only decreased in speed by about 3-4 mph (from 55mph, real speed) by the time I got the the top. No foot pressure was needed, the cruise control had it down pat.

The Bad:

3) the RPM are higher (for the actual speed) than if I had the original tires on. 3000 RPM (@ 60mph, real speed with regular tires and wheels) divided by 0.91 is around 3300 RPM (@ 60mph real speed, with donut tires).

The Questionable:
1) The car's acceleration seems to be a lot faster whether in city or on highway, but again this could be an illusion as I'm technically going slower than what the SG and speedo says.

No illusions. You have effectively geared the car 10% lower. You should expect 10% quicker acceleration by changing the engine revolutions per mile by the mentioned factor of .91.
My winter snow tires have taken me the other way. The OEM summer size is 215-50-15. My soft tread, (high rolling resistance too I presume), snow tires are 205-60-15 or about 14% taller than the speedo and odo were intended to use for accuracy. My acceleration is more leisurely, but my rpm (and speedometer) are lower at a true road speed. It also means my fuel economy has taken a hit because my 'miles' are now 14% longer.
On the plus side my office is now 14% closer to home.;)

[Peakster]

Just came back from a fill and my tank turned out lousy (41.22 mpg). I'd say the donuts for the front wheels are busted. I'll keep them on for a while (I love the acceleration; the Geo can reach 5000rpm in 5th gear with no problem. I also really like the long glides when coasting), but I will eventually put my old winter tires back on. Maybe installing them in the rear would work out better.

[skewbe]

Sorry they arent working, but nice to know that they glide well though. I suppose if you could gear up, you could overcome the smaller diameter and enjoy longer glides, but they are still 3000 mile tires.

How are they at high speed cornering?

[Peakster]

Sorry they arent working, but nice to know that they glide well though. I suppose if you could gear up, you could overcome the smaller diameter and enjoy longer glides, but they are still 3000 mile tires.

How are they at high speed cornering?

I thought about getting 2 more donut tires from a larger vehicle so the Geo's wheel diameter would be the same (or larger), but the 3000 mile life makes it not worth it. I haven't tried high speed cornering with the tires yet because it's still icy outside (Although I once had the Scangauge reading well over 90mph on flat road with them :eek:) .

[red91sit]

I thought about getting 2 more donut tires from a larger vehicle so the Geo's wheel diameter would be the same (or larger), but the 3000 mile life makes it not worth it. I haven't tried high speed cornering with the tires yet because it's still icy outside (Although I once had the Scangauge reading well over 90mph on flat road with them :eek:) .

Perhpaps its the more agressive driving that is hurting the fuel econ.?

[Peakster]

I just put on my 'real' tires back on today and drove the speed limit (62mph) on the highway. After cruising for a while, I reset the SG and was shocked to see that I was only getting 35.6mpg average for that speed. It's just too cold and ugly out (12*F and 20mph winds) to make any assessment. Only A-B-A testing will show for sure.

By the way, there was some noticeable wear on the donut tires when I took them off. The odometer showed 249,053.1 kms when I put the bigger tires back on, so I'll be sure to make the conversions for my next fill, due to the different tire sizes.

[larryg]

Actually, the contact patch is more dependent on PSI than tire size (unless you get really big) next time you look at your tires, notice that they are not perfectly round, they are a little flat where they touch the ground. this is due to the tire being deformed where it touches the ground by the weight of the car. What holds up the weight of the car? the air in the tires, (let it out and see what happens if you don't believe me) The tires deform until the size of the contact patch in square inches X the psi in the tires = the weight of the car.
In fact, you can weigh the car by using 2 sheets of paper, A ruler and a tire gauge. Just put the two pieces of paper under the tire in the front and rear until you cant slide it in any more, then measure the distance between them, do it side to side and measure. the first number X the second number = area of contact patch, multiply by pressure in the tire for the weight on that tire. do all four tires, you get the weight of the car. So, big tires, little tires, contact patch the same, unless you change the air pressure or the weight of the car.

[Peakster]

Actually, the contact patch is more dependent on PSI than tire size (unless you get really big)
...So, big tires, little tires, contact patch the same, unless you change the air pressure or the weight of the car.

The donuts were inflated to the sidewall number which is 60 psi, so they were higher than my "normal" tires that are 44 psi. Also, wouldn't the amount of surface area of the tire touching the ground take into play too (ie. less width to the donut tire)?

[larryg]

You're right the increase in pressure in this case gives you about 1/3 less tire contact area with the road.