Boy, you guys' avatars back to back are trippy!
I have been resetting my "average MPG" and "fuel used" each time I fill up with gas, and so far it is right on the money with manual calculations; i.e. miles driven divided by fuel pumped. If I don't reset it, it starts to differ from manual calculations.
So, now I just reset at each fuel and go with what it says. So far with mixed county road driving and town, it is getting 11-13. We made a trip across WA state towing a trailer and it got 13. The trailer was about 1800-2000 pounds, and speeds were 55-65. We were on state roads not freeway.