Fill the tank.
Drive over 100 miles.
Fill the tank back up, at the same station, same pump. Let the pump shut off at the same time as before. If you finished filling up the first time after the first click of the pump, do it the same this time. Second click, etc.
How many miles did you travel? Say 250. 250/X gallons it took to fill up this second time.
This will be your average miles per gallon, for that tank.
Do you have a long bed or short bed truck? Long bed I think has a 40 gallon tank, short bed should be a 29 gallon.
Just for reference, I just got back from a 4,000 mile round trip to Indiana back to Utah. On the highway, my best tank was 20 even. Worst was coming back, up the mountains- 15.4. Around town I average 16 but I just did the 10K mod and it has picked up 1.5 mpg.