After about 150 tanks in my HCHI, I find that the dash gauge is about 7% high: when it says 53.7 it calculates out (miles driven on tank divided by gallons pumped) to 50.0 mpg, on average. I say "on average" because the variation has been everywhere from 0% to 15% depending on whether I have underfilled or overfilled the tank. And that depends on whether the car is sitting level at the pump, or whether the pump clicks off early or late. I get a pretty close estimate of underfill by watching for the fuel gauge to drop from 20 bars (full) to 19. In my car, that point is reached when I have used 2 gal. from the tank: when I have driven, say, 100 mi with an indicated mpg of 53 or so. If I have gone only 25 mi when the gauge drops to 19 bars I know I must have underfilled by about 1.5 gal.

Taking account of underfill and overfill, my experience is that nearly all pumps I've used in the US are accurate to within 1% or better. Not the case in Mexico, where I have driven extensively on a number of occasions. In Mexico the pumps are as much as 10% off (guess which way!).

Overfill can be as high as 2 gal or more - I have sometimes driven 200 mi before the gauge drops to 19 bars. My experience is only with the HCH, but the underfill/overfill issue probably comes up in all cars, so if you get an indicated mileage that is way off from what you calculate, don't immediately blame the gas station.