My old 99 Classic, w/ Detroit, on board computer lied like a drunken sailer. It progressively increased. 7.5= actual 7, 8.5= 7.5, 10= 8.5. I could always beat the cruise.It was the last of pre lo nox engines. It ended up averaging 6.5, heavy, and through hills, etc. I could get close to 8 on flat, loaded light. Soon as you hit the hills, down it went. No matter what.