I have been wondering about the correctness of the average MPG displays on modern cars. Mine in particular is a Renault Modus. What I did is fill the tank, drive around and then refill the tank to see how much fuel I used.
Based on my calculations I made 557 miles (roughly) on 50 litres of diesel... which I agree is very good and comes down to roughly 50.6 mpg. (using the UK gallon here).
The display, at the time of refuelling, was showing 53.4 mpg average.
My question here is, how do these displays work and how much off can they be? Is it expected to be a good 3 mpg off over a 550 miles trip?