Accuracy of fuel consumption shown on the dash compared to actual

[IAN ROLFE]

If I fill my vehicle and zero the trip, then, refill when close to empty and calculate the LT/100km, I find that the result is at least a 10 % discrepancy between what the car's computer says to my calculation. Well, assuming that the service station's pump is delivering the amount of fuel the pump says, and that the kms travelled is recorded accurately, then how can this be. Does anyone have the answer to this and found the same?

[furtula]

You should never trust it 100%. Even with new cars, the most accurate way to measure MPG is to fill to brim, drive, fill again, and calculate it yourself.

The car computer is not that far off, i found mine to show up to 5% less than actual value. This is with consistent highway driving.

Driving on a mix of urban and rural roads, it can go up to 7% error margin.