Mine under reads slightly, I know I can get 43 mpg if I cruise around 85 down the motor way. Brimming it and recording the litres used compared to the miles traveled though is the only true reading. If your isn't miles off you can use it as a rough guide to see how your drive style affects it. Don't forget there are two settings also, one reads your current mpg but is slow to clock up even if you are economical and the second is your overall average. Once on the mpg setting flick between 1-2 to see how your comparing.
