Mine in stock mode is always within 2/10 of a gallon, with my 80 econo it is off +2, 60 tow about + .5, and with the 120 race I am about where stock would read. This all varies by truck, I have seen some stock trucks off 2 mpg. Try resetting it a couple times. I doubt you are getting 17 with 35" around town with no gear correction. Run it for three or four tanks and check by hand, usually they are off the same amount and then it is simple addition or subtraction off the display number. I can get 15 around town and country driving, and I have a DRW with 4:10's, but I have mods to help with mpg.
NCH