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Old 10-23-2010, 06:54 AM   #11 (permalink)
Tire Geek
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Some thoughts:

1) Differences between tires - meaning make and model - can have a HUGE effect on rolling resistance.

2) Increases in tire size improves rolling resistance, but it's effect is small compared to the difference between tires (See #1)

3) Aero effects caused by tire size differences are even smaller than differences in rolling resistance caused by tire size differences.

4) Changing from P metric to LT metric or Flotation sizing is going the wrong direction for rolling resistance. LT metric and Flotation tires are built out of similar materials and RR is hardly a consideration. If it ever becomes a consideration, they will always be worse than P metric tires because of the loads they are designed to carry.

I support all of that here:

Barry's Tire Tech

So if you are going to report on tire size changes, be aware that the largest effect in rolling resistance is going to be the tires themselves and not the size change. (Please note: I am not commenting on the effect tire size may have on engine rpm and the change in fuel economy derived from that change. I am not aware that there is any data on that!)

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