I have wondered about the accuracy of the fuel mileage function. At home I filled the fuel tank to overflow on a level floor. On the next fuel stop I filled to overflowing on a level pad. I used the low tech method of figuring mpg--pencil & paper. I then compared my results with the DD ProDriver & it matched to a tenth.
Great to know Steve. I have the VMSpc and since it gives you a rolling average it doesn't match the prodriver. But I use the fuel tank gauge on it and it matches pretty darn close to what I actually use. I don't even pay attention to the dash fuel gauge since it's not very accurate.
The rolling average on my VMS only gauges the last 20 miles, whereas the ProDriver tracks it for the continuous trip, so mine don't agree either.