[PaleMelanesian]

When you do a fillup, you can adjust the gallons. It'll apply that % adjustment to the fuel flow numbers from then on. You can change it on the fly within the first gallon of the new tank, too.

When I fill up, I record actual gallons and Scangauge gallons, calculated back to remove the offset. In my case it's +8% gas = -8% mpg. I have a running average for the difference, and use that as my default.

Also, the Speed adjustment also affects miles, so you can lock that in to whatever you want.