Mine over reads by 1/6 pretty consistently, 28mpg indicated corresponds to 24mpg actual. Best real fuel economy I've gotten on a full tank was just over 500miles with an indicated 32mpg calculated at 27.5 mpg (82.5 litres to refill to brim). Mine is not running perfectly well at the moment, I'm definitely down on power but I got that on a single day round trip of 450 miles with another 50 the following day. If you're out on the motorway doing 60ish and you reset it while on the move you will see figures like 40+ until you have to accelerate or go up hill a hill again. Reset it around town and you'll see 19/20 ish indicated.