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One of the problems facing those who want to change tire size in order to get better fuel economy is that a tire's rolling resistance is more tied to the materials the tire is made out of (about a 60% variation) than the tire size (about 5%) - meaning that when someone conducts a test with a different tire size, the tire has to be the same exact model and the same amount of wear in order to eliminate the difference being the make and model of tire. Even tires within the same brand vary quite a bit.
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