Originally Posted by darksky1984

Hand calculating is always more accurate than the gauge inside of your car. The gauge is always 2-3 MPG lower and sometimes much lower than hand calculations.

Originally Posted by Nuke

There are way too many variables. Also, using the miles per tank method is very inaccurate. It's best to hand calculate at every fill-up (and compare that to the on-board after resetting it at each fill-up) and you should have a good idea of average MPG's after a few months.

Not every tank of fuel is comparable to the previous tank so that in itself will skew your data. Add temperature changes, tire pressure changes associated with temperature changes, humidity changes, wind speed AND direction, road surface, # of traffic lights, timing of traffic lights, average speed over that tank of gas, the # of "spirited" trips, and a number of other variables will impact average fuel mileage.