Tire and Rim/Wheel Changes - Overall Effects on FE

[Phoenix]

I am at the bitter end of my tires service life and need to buy new summer tires, and am also considering getting new rims to save on the cost of remounting twice a year... So, now I am feeling drowned by the options and their unknown effects on FE.
Here's how I understand it: The Taller and Narrower the tires, the better, as it lowers engine RPM and overall tire to road friction and therefore raises FE.
Other than possibly not having enough power to drive the larger wheels as effectively (performance impact), I started wondering about other drawbacks that might reduce the potential gains in FE, or even lower it overall.
Here are the factors I was considering in my potential complete wheel replacement, so tell me how you all think that they may weigh in:

Larger Tires raise the vehicle - What effect on FE does the potential increase in aerodynamic drag have?

Larger Tires have more tire to road contact- More Friction?

Larger Tires weigh more- Doesnt spinning weight like that have a significantly bigger impact than stationary weight?

Tire to Rim/Wheel Proportion- If spinning wieght is something to worry about, whats lighter, a huge (17") alloy rim with short sidewalls or a small rim (14") with tall sidewalls?

??? ??? Thanks for the input!

[Phoenix]

OK, I think I have definitely answered my fourth question on this list, and I have a cute little graph to illustrate it. The answer is: that given a fixed outside diameter, it is better (as in lighter which I think I can safely assume will improve FE) and less expensive to purchase, to have as small a wheel/rim as you can reasonably get and and make up the gap with a larger side walled tire.

Damn, how do I paste a .JPG on here if its on my computer?

[SVOboy]

Host with xs.to.

[Phoenix]

oh, sorry, I don't know what that means... Host with xs.to.?

[trebuchet03]

Originally Posted by Phoenix

Larger Tires weigh more- Doesnt spinning weight like that have a significantly bigger impact than stationary weight?

I'm gonna say only when accelerating (that is, speeding up/down, turning). Rotational mass, when constant (steady state), doesn't add anything to the party unless you're either 1) unbalanced or 2) operating at or near a natural frequency

Remember -- the further the mass is from the geometric center (rotational axis and ideally centroid), the bigger change in rotational inertia/momentum. Normally, I'd say SigmaF=ma - but that only applies to "linear sum of forces" -- for rotational mass, you'd use SigmaM=J*alpha where M is the moment, J is rotational mass (dependent on geometry) and alpha is rotational acceleration... I'm not sure if that's the same symbols used in classical physics though (they might use omega instead of alpha and something else for M) :/


Here's a page as it applies to cyclists... Where wheel inertia is a big big deal

http://www.analyticcycling.com/WheelsConcept_Page.html

But again -- for constant velocity... There's zero difference (unless you've terribly ended up in resonance at a natural frequency). But for accelerating.... Given constant mass, go for more of that mass to be near the center of the wheel

[Phoenix]

My "gassaver" education is being applied to two vehicles, one sees a good mix of city and highway driving, although probably more city driving than average (EPA Mileage average that is, we are probably more like 35/65), and the other almost doesnt know what a highway is (more like 5/95)...

So, a think that for me personally that speeding up/down and turning factor in much higher than average and so may be more important than you had thought. I had a little trouble trying to apply those formulas that you gave me (I appreciate the technical and exacting nature of many at this site, but I am unfortunately not as mathematically adept as you ) and I understand the jist of what you can show with the math is that, yes, it plays a role in energy consumption, but I was stumped as to how to apply this to a real world FE change.

From what I gathered in random goggle searches, racers seem to think lighter wheels are noticeably important to power in acceleration, but they are racers, not so concerned with FE and also referring to super light, but also very wide, high performance tires. But, never the less, if you can notice the acceleration difference in the drivers seat of a high torque racer, I would think that a sensitive scangauge would show even more in a small econo box.

only testing will tell, but no one here has of yet, have they?

[CO ZX2]

Originally Posted by Phoenix

OK, I think I have definitely answered my fourth question on this list, and I have a cute little graph to illustrate it. The answer is: that given a fixed outside diameter, it is better (as in lighter which I think I can safely assume will improve FE) and less expensive to purchase, to have as small a wheel/rim as you can reasonably get and and make up the gap with a larger side walled tire.

Damn, how do I paste a .JPG on here if its on my computer?

I use this site, quick, easy and reliable.

http://allyoucanupload.webshots.com/

It would probably help advice givers to know the garage info on your cars. Some cars can barely pull themselves, much less taller tires.

[Phoenix]

Awesome! So thats what hosting is! That was easy,... I'm pretty new to forums and HTML and the like, so thanks CO ZX2.

So I have been doing all this tire research and I had collected all this data to try and figure out what was lighter, a larger alloy wheel with short side walled tires or a smaller wheel with tall side walled tire? I thought it might interest you all...



As for the garage, I will check that out. thanks again.

(new and improved with edit, thanks a third time)

[CO ZX2]

Phoenix. If you copy the bottom link (direct link) after downloading your pic, then you can copy a full size pic into your post. Click "insert image" in toolbar then paste your link into the popup box that appears.

I was in the same boat as you when I started here and got help with the same thing from another member.

[DracoFelis]

Since nobody else has mentioned it, I thought I would...

If you do decide to change wheel diameter, keep in mind that will change the calibration on your speedometer unless you have some adjustment for that (and do in fact adjust for it).

The reason is simple. Most speedometers are keyed off how many times the wheels turn (and just indirectly turns that into how far you went in a given point in time). However, if you put bigger diameter wheels on the car, you will actually go further for each rotation of the wheels (and the reverse is also true, in that smaller diameter wheels move less distance for a given number of wheel rotations). Therefore, the speedometer (unless re-calibrated) will think you are going slower than you really are (or faster than you really are, if you go down in wheel size). And if/when your speedometer is off, you might get an unpleasant surprise from the cops (because you really aren't going the speed you thought you were going)...

So if you are going to go with bigger diameter wheels (or even smaller wheels than stock), you need to be aware that your speedometer will be off (or you will have to recalibrate it to get it accurate again). Not necessarily a reason not to change wheel diameter, but something to think about, if you are planning to do so.

[Bill in Houston]

AAAAaaaand, your odometer would be off as well. It could show something like 250 miles on a fillup, when you really drove 270. Easy to compensate for, but something to keep in mind.

[StanleyD]

Originally Posted by Phoenix

Larger Tires raise the vehicle - What effect on FE does the potential increase in aerodynamic drag have?
Larger Tires have more tire to road contact- More Friction?
Larger Tires weigh more- Doesnt spinning weight like that have a significantly bigger impact than stationary weight?
Tire to Rim/Wheel Proportion- If spinning wieght is something to worry about, whats lighter, a huge (17") alloy rim with short sidewalls or a small rim (14") with tall sidewalls?
??? ??? Thanks for the input!

From what I THINK (Im no expert), but I would reason that larger tires will raise the car and prote more drag under the car. Most people, as I gather, are trying to keep the car nice and low. You are opening up a can of worms when going to a larger tire. Plus you are putting more strain in your engine, gear ratios and the like. My personal, unprofessional and possibly wrong opinion, is to stick with your stock size that you car was built for

WIDER (I assume you meant wider, not larger) tires have more contact and from my common knowledge, skinny tires are more FE and also have less drag. Dont go too skinny and lose control and safety

LARGE sidewall will always be lighter. Low profile tires with tiny sidewalls is all rim which is way heavier than tires. Unless you using some super sweet expensive allow rim then I would assume that steel rim are much heavier than tire material. I would again, stick with stock size, or MAYBE go a little skinnier.

Get good quality tires with decent LRR numbers and moderately increase psi unless you are kamikaze daredevi and willing to try anything. Remember that tires built just for LRR wear very fast. Lots of really angry Pruis owners changing tires after 20K miles

[s2man]

Pheonix, I've just been through this tire-size decision process, and I can confirm that you can be drowned by the options. Tire width and height, rim size and mass, rolling resistance, traction, speed, load and temperature ratings; there certainly are a lot of variables.

Trebuchet and the racers are correct that less mass is easier to accelerate. But I ruled out light-weight alloy wheels, as it would probably take forever to recover their cost in fuel savings. So what is left for FE? Friction, aka rolling resistance. And as you and StanleyD noted, skinnier tires are usually more FE. So I looked for skinnier tires with a low rolling resistance.

DracoFelis and StanleyD's comments about changing the tire diameter are significant, because this also changes your effective final gear ratio. I don't have overdrive, and I considered larger diameter tires. But I decided to keep my tire diameter the same and only change the tire width. ( I am double thinking this decision, as changing tire size is much cheaper than a gear or transmission swap to achieve higher gearing... but too late)

My Cavalier came with 195/65R15 tires (the "big 15's") though most Cavaliers came with 14" wheels. I could not find 185/70 15" tires, but I did find 185/75R14's. They are not common, but available online. This size maintains the same diameter as the stock tires within 2 rev's per mile. I found 175/80r14 tires online in Europe, but not in the USA. This is interesting to me since they are more into mileage than the U.S.A. See the pictures below for a comparison of my old and new tires. Remember, the new tire is not inflated, so were not comparing apples to apples here. But I wanted to show them anyway - they look like bicycle tires compared to my old tires. I hope, when mounted, they maintain that rounded cross section at the tread, instead of being flat like the old ones. That would reduce road contact even more. I'll post another shot once I get them mounted.



These are the General Ameri G4S. I only found two sources of RR data online. The data from 2002 lists the Continental Ameri G4S as having the second lowest RRC at 0.0078. Since Continental makes the General tires, I'm hoping it is the same tire with just a different branding slapped on. I've written Continental asking if this is the case. My tires have a treadwear rating of 520 and a 60,000 mile tread warranty. So they should last a little longer that the Prius OEM tires, which have a treadwear rating of 160.

When I tell my friends that I got some tall, skinny, LRR tires, many respond with "won't that affect your traction". Well, here's where I get to rant. <standing on soap box> The current fad of wide, low-profile tires is silly. That style of tire is only good for traction on dry pavement. ie, racing or trying to pull 1g in the turns. Narrow tires concentrate the weight of your car on a smaller footprint, giving better traction in rain and snow. And THAT's when I'm concerned about traction. < off the soap box now> The 195's on my Cavalier hydroplane easily and were terrible in the snow. I expect much better performance from the new, narrower tires. I will report my findings as soon as I get some 14" wheels.

[baddog671]

Keep the original size tires and wheels, but get aluminums. It'll stay correct and have less rotational mass.

I saw a set of wheels and tires on ebay the other day, we're 8 pounds each(that's wheel AND tire).

[s2man]

Update: Continental replied that, yes, the General G4S's are the same tire as the 2002 Continental G4S. Woohoo! I've got some of the lowest Low Rolling Resistance tires out there. Go Team OPEC Busters!

[VetteOwner]

i noticed absolutely no difference in MPG switching from a WIDER tire from my stock ones. exact same rim side both steel rim one set is chrome plated tho. same tire sizes except one set is slightly wider by about an inch or 2

[Wazabi Owner]

I just switched from the stock 14 steelies with racing disc hubcaps (threw 1 due to the cr-p road I live on) to 17's on my Corolla. Gas mileage has taken a SERIOUS nosedive. I'm flat out embarrassed. But dang, the car looks sooo good - I still think I will have to be admitted if I continue the horrible mileage. Good thing is the rims will fit whatever car I have, I may swap them to whatever is the least driven car. Actually, the Corolla may be driven less very soon. I'm almost done building my other car.