Thats what I do, I reset the trip meter, fill the tank and when the needle is on the full line, reset the meter again. The tank is 55L so it costs just under £50 to fill from empty so thats how I know how much fuel is going in.

I am never going over 55-60 mph so it isnt being thrashed.

The thing that was replaced on a friends car is a part of the engine management system.