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The mass moment of inertia of a disk (I) is equal to 1/4 mass times the radius squared. The stored energy in a rotating disk is equal to the 1/2 moment of inertia times the rotation speed squared, which is equal to the vehicle speed divided by the wheel radius. Thus for a given speed, E = 1/8 m * v^2 or in English the energy stored in the wheels is independent of radius for a constant mass. If the bigger tires are heavier, then the stored energy is proportional to this increase. For stop and go driving this is not good, but for me experience with bigger tires saved gas due to the lower rolling resistance, lower manifold vacuum, lower engine friction, lower drive train friction and lower brake drag. Of course you will never break even with new 17 inch rimes. MY experience is your MPG gain will be approximately half the tire diameter ratios. If you put 5% bigger diameter tires, MPG will increase by 2.5%
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- Tony
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