Mine seems to be accurate.

The way this works - I think - is that the computer for the fuel injection measures how much fuel is being metered into the engine which then compares this amount with distance the car has traveled. If the engine is running properly then you would think this would be fairly accurate but I've seen other complaints. There could be a lot of variables with doing your own calculations such as temperarture, the pump at the gas station, how consistent you fill your tank to the same level, etc.