Installing Bigger Wheels

[suprapsu]

I have a 1994 Honda Civic DX 4 door. Right now it currently has 13 inch wheels. By installing 14 or 15 inch rims/tires would it improve my fuel economy?

[omgwtfbyobbq]

You need a tire/wheel combo with a larger outside diameter than what you have now. Your car will be slower through all the gears, but on the highway, your engine speed should drop and efficiency will increase by a little bit.

[suprapsu]

My little brother had a 1992 Lexus SC400 and he upgraded from 17 inch rims to 20 inch and he said it killed his FC. Maybe this might be because the 20 inch combo he got was alot heavier?

Also I heard, I'm not sure if it's true but the speedo and odometer gets an inaccurate reading. IS this true?

[omgwtfbyobbq]

Yes, the speedo/odo can change if the OD changed, if that size is a stock option, you can generally get the right speedo/odo components so the readings will be correct. When people go from big rims to really really big rims, especially on an auto, it can increase the amount of time spent downshifting and keep the tc unlocked which can hurt mileage. In any event, there's only so much you can fit in wheels wells, so the really tiny tires that go on really big rims probably don't have the best rolling resistance. What was the difference in OD going from 17s to 20s?

[MorningGaser]

going up in wheel size may well decrease the torque at the wheels, making it harder for the engine to turn the wheels, meaning an increase in more gas burnt, lower MPG, etc. However once at highway speed, the engine turns less times for a given speed, so this issue is not linear, to be sure. Perhaps in city stop and go you'll get less MPG, but perhaps more on the highway.

There is a relative maxima in the MPG curve: as the tire size increases, MPG increase but to a certain point in the curve, then increasing the tire more will lower MPG...there is a sweet spot in tire size.

I think 90% or more of the time, people that put the over sized tires on their vehicles will actually experience less torque at the wheels (power) and less MPG....it's just for show...just like most of those wings you often see the backs of cars....these give no benefit...just for show...I would suggest sticking with the stock factory sized wheels, but I could be wrong on this.

[Erik]

I tried putting larger tires on my 87 Acura Integra and was surprised to see that it decreased my mpg. It may have something to do with the car riding higher and allowing more air to travel under the car.

I would guess that each vehicle is different and that it may help some and hurt others depending on aerodynamics, torque etc.

[MorningGaser]

Originally Posted by Erik

I tried putting larger tires on my 87 Acura Integra and was surprised to see that it decreased my mpg. It may have something to do with the car riding higher and allowing more air to travel under the car.

I would guess that each vehicle is different and that it may help some and hurt others depending on aerodynamics, torque etc.

Bigger tires mean less torque at the wheels, which means more engine effort to turn those tires, which can translate into less MPG.

[Sludgy]

Are the bigger tires are wider or taller, or both? Narrow tires will tend to give better mileage. Wide tires will give worse mileage:

Wide tires have more tread on the road. Wider tread means more tread deflection, which increases rolling resistance.

Wide tires have more frontal area, increasing wind resistance.

Wide tires and some rims have terrible drag coefficients.

If you want better mileage, install SKINNY tires on stock rims: I put skinny 235/85R16s replaced fat 265/75R16s. I'm now getting close to 20 mpg out of a 1 ton pickup.

The best of all worlds is to install tall & skinny tires. Taller tires would decrease engine rpms and use less gas. However, tall / skinny tires and suitable rims can be very hard to find, since car tuners all want the pimped look.

If you make a tire / wheel change don't forget to adjust your speedometer for the different tire diameter.

[Erik]

It seems that manual transmission swaps (switching to a trans with different gearing to lower cruising rpms) help most cars' mpg. I believe that my swap helped my little 87 Civic (I switched to an HF trans from an 84 CRX- the one that was rated for 50 mpg on the highway).

In a way, the trans swap also lowers torque at the wheels (by having a "higher" gear). So it seems like using bigger tires is simular to a mini trans swap that also messes with the aerodynamics.

I know several of us have done a trans swap and all the news I have heard has been good- although in theory a person could be hurt if their engine didn't have a high enough torque peak at the rpm that they would use on the highway. From what I have read here, it seems like there have been mixed reviews when it comes to increasing tire size.

[Bill in Houston]

AND, I think that for a given tire diameter, a smaller wheel diameter gives better mileage. But I can't prove it.

For example, if the max that your car will allow is a certain 215/80R15, then that will have lower rr than the same diameter 215/??R18.

I think that it has to do with the sidewall stiffness...

[Brian D.]

Suprapsu,

I'm currently doing this with my '89 CRX HF. I'll let you know how it turns out. I'm swapping out my stock/OEM Honda 13" x 4.5" steel wheels for a set of the superlight Mazda Miata BBS 14" x 6" rims. I've already had the center bore on the Miata rims machined to fit the Honda hub. Now it's just down to ordering the tires. ...and I've just posted that question here on another thread. I'll keep you posted as to the changes.

***However, I'll mention in the meantime that I'm currently experimenting evein before installing the Miata rims. I've got a random set of '93 Mitsubishi Mirage steel rims (13") with 185/80-13's on the CRX HF right now. I've been driving with them on the car for about 2,000 miles now. They're heavy. And the 80 aspect ratio is ugly. I think the sidewalls on this no-name tire flex a lot. They grip great, but aside from that the car is a pig as far as handling now. My speedometer, with these large tires, is off by approx. 5 miles per hour...which I LOVE! Finally I can go the same speed as other cars without my RPM's shooting through the roof. I have it all figured. My mileage multiplier is 1.0741 (so if my odometer reads 300 miles, I've actually gone 322.23 When I use this to figure my mileage, there is a definite increase in Highway MPG. I'm just not sure about city MPG yet.

[omgwtfbyobbq]

Originally Posted by Bill in Houston

AND, I think that for a given tire diameter, a smaller wheel diameter gives better mileage. But I can't prove it.

For example, if the max that your car will allow is a certain 215/80R15, then that will have lower rr than the same diameter 215/??R18.

I think that it has to do with the sidewall stiffness...

What you're referring to is the tendency for manufacturers to make LRR 13/14/15" tires. Most 16"+tires usually aren't marketed at the economy crowd, so there's not much LRR up there, as well as having a smaller sidewall to tread ratio like you mentioned. In any event, assuming rolling resistance is the same between tires, going w/ a bigger tire/wheel combo should help out efficiency a bit.

Originally Posted by MorningGaser

Bigger tires mean less torque at the wheels, which means more engine effort to turn those tires, which can translate into less MPG.

It's usually the opposite, all things being equal. the more the engine works at a given speed, the greater it's efficiency, up to a point.

[Phoenix]

Ive been researching this myself recently and have come to the conclusion that the smaller the WHEEL(as in the metal part) the better (assuming that the outside diameter of the tire stays the same,) so if FE is your goal, stay with the 13" wheels. This is because that given a fixed outside diameter, increasing the diameter of the wheel adds proportionately more weight to the wheel/tire combo than add more sidewall to the tire. Keeping the weight down, especially unsprung weight, and rotational weight, of which the wheel and tire are both, has a much bigger impact than the same weight in the trunk.

If you do want to try lowering engine RPM via larger outside diameter, than just buy larger tires for you existing rims, but this raises some of the same questions I asked about in another thread. If I new how to link the thread I would, but I don't so just find the one in this same forum category called "Tire and Rim/Wheel Changes - Overall Effects on FE"

[Ryland]

if you are going for better gas mileage, and I assume that you are, I would stick with 13" rims, because it's really hard to find narrow tires for larger rims, finding 4.5" wide rims is almost impossible, they only came on a few of the pre-1988 civics as light weight steel rims on things like the HF, and standard on the pre-1984 civics, weighing around 12-13 pounds each, or as civic vx alloy rims weighing 11pounds each, a 13" rim can take a 155mm wide tire, where the narrowest you can get in the USA on a 14" rim is 165mm, but with limited options, 15" gives you even less selection.

[kickflipjr]

I think I smell an experiment brewing.

I believe that changing to 13" to 15" wheels would show a increase in mpg (just remember to set it so the odometer wont be off).

[Ryland]

it really depends on your tire profile/sidewall hight, the larger wheels tend to be wider, and thus weigh more, low profile tires don't tend to weigh less, so you are increasing your rotating mass, and eliminating your option of usieng a narrow tire that has been proven to roll easier, and weigh less.
want an experiement? find simaler weight tires that are simply taller.

[Brian D.]

Did no one read my post? I mentioned some of these things that are still being discussed.

With each engine having its own speed/RPM range for peak efficiency, if I start to get lousy gas mileage at a lower RPM on the highway (with my actual speed being the same) then I can assume that I've now got to drive faster to get my car back into that peak RPM/efficiency range which used to be lower due the smaller diameter wheel/tire combination that I had on the car with my stock HF.

[omgwtfbyobbq]

Oh yes, now that you mentioned it, imle cars built during the 80s, and even some economy cars in the 90s, have nearly optimal gearing for FE assuming stock configuration. Otoh, cars a lot of built in the 90s are not in the BSFC peak for their engine/car/transmission. YMMV... It's best to find a BSFC map in order to figure out what will be best.

[VetteOwner]

hmm well my car is 27 years old (1980 chevette) and going at highway speeds (55mph) its at 2800 rpm@55mph. at 60 its 3K and ive driven on the interstate once for about a mile with it to see what the rpms would be. lets just say its not pretty. this car was obviously geared towards mostly city. i need some new tires desperately but im a tad concerend about having them too big. the current tires are 165-80R13 i was thinking maybe 195-80 R13 as i do mostly highways speeds (45+) but some city where it can be conjested but usually can cruse easily at 35-40 mph.

am i correct that those tires i plan to get will have the same width just a taller sidewall?

[Bill in Houston]

Originally Posted by omgwtfbyobbq

It's best to find a BSFC map in order to figure out what will be best.

Where would I look for one of those? Just searching in Yahoo didn't really give me good results. I am most interested in my k24 engine in my Element... Thanks!

[omgwtfbyobbq]

It's kind of a PITA, but googling has helped me find two out of three engines I was looking for. To give you an idea of how much engines haven't changed, a ~30 year old passenger diesel engine will be most efficient at ~2400rpm/80% throttle, a twenty year old gasoline engine is ~2200rpm/80% throttle, the Prius' engine is at ~2700rpm/80% throttle iirc. It's pretty much in between 2000-3000rpm, and near full throttle. However, gearing the car like this would result in crappy gradability/acceleration in top gear, so imo, about half throttle at whatever speed you want to cruise at is generally a good compromise. For my Camry, dropping engine speed by 25% would increase mileage by ~10mpg at 55mph and I'm guessing you'll see something similar for your element. I like to use a ratio of engine displacement to speed at ~55mph, so my 3L engine should be turning at 1500rpm there, a 1.5L engine can turn at 3000rpm, and a 6L engine should be idling down the road for pretty good mileage. For your engine, I'd guess ~1900rpm (edited for error) is good, but... I wouldn't actually recommend it until I had some more info on SFC.

Edit- Your gearing should be .825/5th and 4.765/r+p, so with 215/70R16s you should be o.k. at ~2600. Of course, the automatic is at ~2175rpm at the same speed, so I'm guessing you can safely go down to there, or even 2k rpm, since the automatic gets the same highway mileage as the manual because of it's lower ratio, a manual w/ that ratio should break 30mpg np. A transmission out of a 5spd accord should put you at 34mpg EPA highway, if it'll fit.

[Phoenix]

Originally Posted by omgwtfbyobbq

When people go from big rims to really really big rims, especially on an auto, it can increase the amount of time spent downshifting and keep the tc unlocked which can hurt mileage.

What's tc?

[omgwtfbyobbq]

Torque converter clutch. Normally automatics are connected to the engine all the time through a fluid coupling called the torque converter, which is why when stopped, they can idle in gear w/o stopping. The engine just spins the fluid around w/o stopping. If it were a manual transmission, the car being stopped would make the engine stop because there's a direct manual connection. In an automatic, the tc is a mechanical connection that will engage/lock up when certain speed/engine temp/etc... conditions are met to improve efficiency. It's better to drive w/ it locked up as often as possible because you get an extra ~10% (or more) boost in efficiency compared to normal operation w/ the torque converter.

[Bill in Houston]

Originally Posted by omgwtfbyobbq

It's kind of a PITA, but googling has helped me find two out of three engines I was looking for.<snip>.

Thanks bbq. I'll keep looking. There's gotta be one out there.

Here in the flatlands, I could bump up the final drive AND add a sixth gear, and I think it would work wonders. But I don't think there's a payout, since I am not able to do it myself... But I think the E would gladly do 60 mph at 2000 rpm. It's a pretty grunty engine. I will keep scanning the horizon and looking for opportunities to make it happen...

[brucepick]

Tire size calculator:
http://www.wickedbodies.net/Tire-Size-Calculator.htm

Tire's overall outer diameter is what will affect engine rpms at any give ACTUAL road speed and thus your FE.

You can change the rim size and get the same overall diameter or not, that depends on the actual tire size you use.

IF you change the overall diameter (which is the only way to change engine rpms using the tires/wheels) then your speedometer will be a liar and you'll need to compensate with different speedo components etc. That may or may not be possible to do. If not, you risk speeding tickets. Not worth it in my book, ymmv as usual.

Rims also have a flange width dimension which affects the range of tire sizes it will accept. See tire specs at tirerack.com for details on flange width.

[VetteOwner]

hm i did that tire size calc to the tires i have and the tires i want.

turns out that my spedo would be off by 3.5 miles(it reads high anyways and this would line it up perfectly)

there would be 45 revolutions of the wheel less per mile with the new size

the tire height diameter would be 1.26 larger

which isnt all that much bigger but i belive it would help. (still only $32 a tire)

[brucepick]

Originally Posted by VetteOwner

hm i did that tire size calc to the tires i have and the tires i want.
turns out that my spedo would be off by 3.5 miles(it reads high anyways and this would line it up perfectly)
there would be 45 revolutions of the wheel less per mile with the new size
the tire height diameter would be 1.26 larger
which isnt all that much bigger but i belive it would help. (still only $32 a tire)

VetteOwner,
Good work.
Did you check the door sticker to check if you're running OEM size tires now? Could be that speedo is off due to a different tire size on car now.
Or maybe rear end or tranny was changed at some point??
Anyway if car is set up as originally, I'd check proposed tire size vs. OEM size and see how that comes up on the calculator.

[VetteOwner]

no i broke the origional spedo that used read correctly but i had to ge one out of a different yeared vette. it has the stock tire sized on it now. (maybe the origional tires who knows, cuz thier pretty bald with low miles)

it is and should be set up as factory. i havent seen any evidence of repairs anywhere on it.

so im guessing that the tire diameter being only different by 1.26" would make a bit of a difference (this would accualy correct my spedo being off) since i gotta get new tires anyways i might as well go a bit bigger since i drive 60-75% highway (45-55mph)

[Wazabi Owner]

I did it with my Metro, went from stock to bigger wheel/tire combo and it KILLED my mileage. Check my garage and you can see pics of the wheels. Went back to stock 13's w/racing discs. IMO, the optimum choice. Only thing better would be to get a set of RE92's to roll on. In time, I'll do that.

[Phoenix]

[quote=Wazabi Owner;50807]I did it with my Metro, went from stock to bigger wheel/tire combo and it KILLED my mileage.QUOTE]

What size were the new wheels(rims)?

What size and type of tires did your 13" setup have?

What size and type of tires did your larger setup have?

Did you happen to wiegh the complete unit and compare it to the 13" setup?