Yeah hand calculate and see. Mine has read 90mpg before after resetting it. Fill it up, drive half a tank and fill it up again, then hand calculate and see what you get.
2003 F-250 CCLB 7.3L
4" lift and PMF traction bar/block setup running 35s on 20x12s. Intake, exhaust, PHP Hydra w/ Gearhead tunes, gauges
2005 F-350 CCLB 6.0L
6" lift on 20x10s and 37s, studded, deleted, Powermax and Livewire with Quicktricks tunes, exhaust, intake (Wife's Truck)