[Blue Bomber Man]

I really think that the analysis of whats more efficient needs to broke down into two catagories:

1) Engine Efficiency

2) Tire efficiency

So with 1) yes a larger tire means lower RPMs and thus is likely to be more efficient at the engine. However we also know that increasing the diameter would increase the rotational momemtum at the tire. But something that is being neglected is that conversly you are LOWERING the rotational momentum of the engine, transmission, and power train. I would speculate that the larger tire mass is not worse than the gains in the rest of the car.

Now of course as MetroMPG has done you could change your gearing and thus gain the advantage at the engine without having the negative effect at the tire....

But it has been shown by a survey of a 150+ tire models that on average size 15-16" tires have better RRC (rolling resistance coefficient). So if you compare say a car with 12" tires with a higher RRC to the same car with 15" tires that have a lower RRC, and equalize the gearing so that the RPMs match for any given speed, the car with the lower RRC run more fuel efficiency.


What effects RRC?

1) Tire pressure, as we have mostly all accepted raising your tire pressure significantly improves gas mileage

2) Tread depth, the less tread a tire has, the lower its RRC. A tire's RRC value actually improves with age, so suck the life out of those tires as long as can safely be done.

3) Tire width, narrow tires will out perform wider tires in general

4) The remaining properties that effect a tires RRC include: composition, sidewall strength, sidewall width, and road conditions.