[LuckyRVA]

Quote:

Originally posted by Wynn
The inertial difference associated with getting larger wheels rolling should have a negligible difference in mileage; once spinning, the larger wheels are at no disadvantage in efficiency.

You forget about wind resistance and friction.Larger hence wider tires will have more of both, especially at higher speeds, it's called rolling resistance. Rolling resistance can have a huge affect on gas mileage at high speeds. Thats why all of the economy cars come with tall gears and skinny tires. ie Toyota Prius and Honda Hybrid.
Rolling resistance actually takes away on average of 4.2% of the power the engine makes, pushing the car against this force results in higher gas mileage and the bigger the surface area; the more resistance.