My cars avg L/100Km is WAY more optimistic than calculated...

[wungun]

Been gauging my averages using Fuelly, and notice at times the readout averages in my cars built in mileage computer doesnt always match up with the calculated Fuelly numbers...

Like the fill up I did today...car said 5.9L, Fuelly said 6.5L

What gives...?

[F8L]

Many cars do not calculate mpg perfectly and even more commonly, you may pump more or less into the tank at each fuel up. This will skew individual tank calculations but over the long term with enough refill data you were average out these discrepancies.

For example, you may have added more fuel on this last fuel up. That will make this tank appear worse than it actually was. At your next fill up, assuming you stop the pump at the first click, you should see less of a discrepancy which will make that tank appear better than it was. The two average each other out.

It is very important to use the same exact pump and stop pumping after the first click if you are going to monitor the difference between car calculated and manually calculated fuel economy.