[Big Dave]

I've extensively studied this on pickups. choosing the optimum tire size is something the factory boys generally get right.

Remarks refer to driving wheels.

Go up in diameter and you lose MPG (even after recalibrating your speedo). Larger tires have a larger rotational moment of inertia and you have to expend more energy to accelerat them up to speed. Not a big deal if you exclusively drive the slab, but murder in town.

In a perfect world you could use the increased rotational energy in your bigger tires to coast further, but there is way too much traffic in my world for that.

Go down in diameter and your engine sppeds up and has to expend more fuel to overcome engine frictional HP.

With my 4x2, I split the difference. Kept the rear (drive) tires the stock size (235-85x16E) and reduced the front tires to 225-75x16E to lower the front and additional inch.