[piecesparts]

You need to look at your tire size, rear end ratio, and RPMs for the speed you choose. Once you know these, then you can figure closer what your mileage should be. If your tires are TOO tall, then you will work your truck to death getting up to speed and overcoming hills. If your tires are too small, then the RPMs will be high. Changing tire size is the equivalent of vchanging rear end gears. If you are geared too low, then you will run high RPMs and the mileage will be bad. If you are geared to high, the engine will work harder in a city type of driving, but will be smoother on the highway.

Is your exhaust and air cleaner restrictive on flow? That will change gas mileage a lot. Getting rid of the stock belt driven fan is worth a single MPG gain.