Hey Skeeter ,, welcome to the forum ,,
U did not state what engine u had ,, or if it is a DP or a Fred..
Sorry ,, DP means ,,, engine in back and Fred is engine in front ,,,
but let us know on u'r rig and also what size is it ...
1) Fill up the tank
2) Drive a known distance (say 100 miles) at a constant speed. 55 would probably be best for best mileage, but drive whatever speed you plan to use the most.
3) Fill up again. Best would be to use the same pump at the same station.
4) Divide the miles you traveled by the number of gallons from step 3.