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Old 12-05-2009, 08:07 AM   #6 (permalink)
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Originally Posted by winkosmosis View Post
I'm skeptical about the impact of wider tires on fuel use. A wider tire with the same contact patch as a narrower one is also deforming less, which has to count for something.
Me, too! But rather than just wonder, I pulled out my Excel spreadsheet and did some calculations.

According to the first post (not sure where the data came from!) The average for a 10mm increase in tire width was 0.003 or about a 10% change in Cd.

According to US Department of Energy, the effect aero has on the EPA fuel economy test is 3% for the Urban cycle and 11% for the highway cycle. (For rolling resistance of tires it's 4% / 7%)

That means the effect on fuel economy of a 10mm change is 0.03% to 0.11% - wider being worse.

Smithers reported to the California Energy Commission on a study of tire sizing and its effect on RR. From that data, the effect a 10mm increase in width has on RRC is about 3% (if you assume there are no other changes)

So if you combine that with the effect RR has on the EPA test, then a 10mm increase in tire width DECREASES the fuel economy by 0.12% to 0.21%, which is significantly larger than the effect caused by aero (0.03% to 0.11%)

This means the aero effect of the width of tires is more than offset by the improvements in RR.

This may seem counter intuitive as wider tires ought to have more RR, but most of the effect is coming from the less deformation as winkosmosis suggested.

Last edited by CapriRacer; 12-05-2009 at 08:23 AM..
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