[Big Dave]

In the pickup community, a lot of guys have tried bigger diameters tires and the result is always the same: lower MPG.

The rotational inertia of the wheel tire assembly goes up with the square of the diametyer. Every time you accelerate the vehicle you have to not only translationally accelerate the vehicle but also you have to rotationally accelerate all the rotating machinery including the wheels and tires.

So everytime you pull away from a stop bigger tires have the same effect of alot more weight. Very few guys can drive so as to recover all that energy by coasting.

This is why very high-MPG cars come with preposterously small wheels and tires.