If your speedo has not been recalibrated then the computer wont read mileage properly either. So if you havent had the truck speedo recalibrated your actually seeing that you are driving few miles than what you actually are along with the speedometer reading you are going slower than you actually are. Hand calcualtion cant account for that unless you are converting your miles to compensate for the bigger tires before dividing it by fuel put in.

how old are the injectors? i have a 3" body and a 2.5 leveling kit on mine with the millitary 37. i was getting 24mpg on high way and my milage droped about last summer it kept getting wers and wers. then i lifted it to the 3 and 2.5 and lost 2mpg i went from 16 to 14. i was pissed that it was at 16 with stock tires and height. i am now getting 9.8 on avg. so i said enough is enough and i looked over my truck and found that 4 out of the 6 injectors were nasty and closed up. so i was just domping fuel in insted of spraying it in. i have put stock injectors in (my brothers old ones) and i now get 18 mpg again. but i am going to get some marien one for 60 hp gain and 2-3 mpg gain.