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Originally Posted by Green Machine
If everything else was the same, tire width and over all height, which tire would roll easier, a 15 inch wheel with a 75 aspect ratio or say a 17 inch wheel with a 60 or 50 aspect ratio tire ???
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I would guess they would be about the same assuming they were both the same weight. If one weighed more than the other it should get slightly worse MPG but I can't imagine it would be much and probably wouldn't be measurable.
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Originally Posted by Green Machine
Also, related but not the same, if geared so cruise rpm is the same, would a smaller diameter tire or a taller diameter tire roll easier ?
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The shorter tire will roll easier because it has less inertial mass to overcome for it to spin.
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Originally Posted by Green Machine
I know that wider rolls harder too, but is it measurable mpg difference ??
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If you're going from a 235/75R15 to a 255/70R15 (same height but 20mm wider) theoretically there should be a difference. However, I've done this swap multiple times ('94 Burb, '86 GMC and Old Yeller) but in all my vehicles I've never been able to measure any difference. You would probably have to go to a much wider tire to notice a difference.