If you have access to vagcom you can calibrate the trip computer to be pretty accurate, I've got mine within 0.5mpg don't think its possible to get it much closer unless you have a more accurate method of briming the tank right to the top.
I only have the trial version, I'd be too worried about mucking stuff up with the full one

TBH switching the trip comp MPG readout is the best way for me. With it switched on eveytime I nail it I think "oops"


