Larger wheels for MPG?

[UFO]

A shorter sidewall on a tire of the same outside diameter should slightly reduce rolling resistance?

[abently]

I changed to slightly taller tires (7% more circumference) and didn't notice any great change in FE.

I think header design/torque curve has more to do with how well such changes work.

[Vekke]

My cars stock size was 185/65R14. Every time I put on my toyo 350 175/80R14 my gearing is about 9% taller. Also my fuel economy becomes about 9% better. So in my 1.9 TDI Seat taller gearing works very well. Best way to test this is to get used tires from local tire shop for free and just test it. No sense to buy new tires in my opinion. Also best way to see if any changes will work is to make it so big that effects will be propably be big if it works. Otherwise you need to drive a lot to see the difference. On my case I notice it from the first tank when I put the tires on .

One thing that also counts is that tires should not weight much. I have also tested 1% even taller tires and got much worse fuel economy. So if your first tire does not work it does not mean that it wont work with some other tire. I would recommend that Toyo because it has worked like dream with my car.

[zonker]

in theory, assuming all other factors are equal (which they rarely are), the best selection is the smallest rim diameter with the tallest tire.

rim diameter: the heaviest sections of a wheel are at the rim, and the heaviest section of a tire is at the tread, so lower profile tires and larger diameter wheels, given the same tire outside diameter and total weight, will require more energy to rotate.

so in the event of adding a larger diameter tire than what you have:
if there is a way to move the heaviest part of the rim further from the tire OD (smaller rim diameter) it would help offset some of the flywheel effect you will get from a larger diameter tire.

from my own personal and very recent experience, my 2.3 mustang was originally shodded with 195/75-14 tires and polycast wheels (steel rim with a alloy looking plastic molded over the steel). that combination weighed 36 lbs. In a pinch, I had to replace the tires with 215/70-14s on the same rim, tire diameter increased under a half inch, but tread width grew and weight jumped 2 lbs. my previous best mpg was 41, but now my best is 36. I'm not saying the 5mpg drop was solely tires, but my guess tells me they did contribute.

my mpg tire/wheel plans have me looking at air cooled beetle tires (165-80-15) mounted on factory mustang cobra alloy spare rims (15x4, 4x108, 28mm offset, 8 lbs each). i should shed over 10 lbs a corner, reduce my contact patch and still maintain the stock tire diameter. I'm sure my tradeoff will be marginal stopping distances and tire adhesion, but I'm willing to accept that in pursuit of eco efficiency since i do not drive this car at 10/10.

[SentraSE-R]

Originally Posted by Frank Lee

We need to watch how that toothy green box out in Cali does with it's new shoes...

I'll probably start a new thread when I do my ABA testing, because it gets complicated fast. I drove 800 miles last weekend. New tires calibrated 9.9% lower than the odometer, per my GPS. I did a couple of 60 mile, and one 80 mile, leg(s) for calibration. My first tank was mixed - ~40 miles on the stock tires, 100 miles on the new tires uncalibrated, 170 miles on new tires calibrated to 5.5%, and 180 miles on new tires calibrated to 9.9%. I had to rely on the Ultragauge's distance traveled, as I accidentally reset the car's trip odometer. Bottom line, a really questionable 53.7 mpg for 514 miles, my best highway trip in my fuel log.

The new tires, however, are not 9.9% taller. They're only 5.31% taller than stock, and roll 5.605% less distance. That means my baseline with the stockers is flawed. I never calibrated distance with the stockers. When I do, I expect there'll be a 4.3% odometer error. That means instead of averaging 47.1 mpg for 20,000 miles, I actually averaged 49.3 mpg, and my impressions of what is normal are not normal.

My second tank was a short one, just 174.2 miles on 3.653 gal = 47.7 mpg. That mpg falls inside the normal mpg I got with the stockers on highway trips, adjusting for the 4.3% odometer error.

Right now, it looks like the new tires will be close to the stockers, despite weighing more and having worse aerodynamic properties.

[Vekke]

Going to wider tires will hurt your CW-value. 20mm more width is huge increase. So like Zonked said. You can also find taller tires that are even skinnier. If you put there 3.5 bar or 50 psi they wont be wobbly even at pretty hard cornering, I can promise that. Also new tires dont roll so easily than the worn tires so that is why I recommended to test the effect on used worn out tires. Now I might see little drop on my fuel consumption with new tires, but well see that in the next month.

[CapriRacer]

Perhaps this will add to the discussion:

Barry's Tire Tech

Bottomline: They are a lot of factors that impact RR and tire size is small compared to the differences between tires. New ns worn is another. So when you do A-B-A type testing, you need to be aware that tire size is just one of the factors - and may be not the most important one.

[SentraSE-R]

My personal comparison will have too many variables to answer the taller tire question, The best way to test just taller tires would be to find another xB owner with stock steelies, buy taller tires of the same brand and model for my old wheels, wear them in, and then do the ABA testing with the tires from the other car to swap between tests. That would be hard to pull off logistically.

But I want to answer the basic question for myself, even knowing I'm throwing in different wheel sizes, wheel aerodynamics, heavier tires, different inflation pressures, effective gearing variation, different tire compounds, different tread patterns, wider tread width, etc. Most of the significant differences work against my new wheel/tire combination. I can still pose the question: do taller tires improve mpg despite all these other factors stacked against them in this testing regimen? Actually, I guess the question is: what results will I get with my taller, heavier, less aerodynamic tire/wheel combo compared to stock?

[zonker]

Originally Posted by SentraSE-R

I can still pose the question: do taller tires improve mpg despite all these other factors stacked against them in this testing regimen? Actually, I guess the question is: what results will I get with my taller, heavier, less aerodynamic tire/wheel combo compared to stock?

taller - improvement
heavier - loss
less aero - loss

the vote is 2-1, you'll lose mpg

[arcosine]

My car gets better mpg with taller tires and wheels, FACT, not opinion.