Computer miscalculates fuel consuption?

[bletour]

Hey all,

First I'm fairly new to the forum after recently picking up a 2006 4.4i. So far a few minor things (steering rack/pump noise, rear hatch thumps, power mirrors not folding, rattle in driver door) but the X5 is fun to drive for such a big heavy car.

This forum has been a great help with some of the issues (I just did the ATF fluid swap for the steering rack and the noise has been cut in half. I hope the next drain and swap will fix it totally) so I was wondering if anyone has seen this issue.

The "Consumption" MPG reading is WAY off. It has been telling me 22-23 MPG regularly but doing a gallons to miles manual calculation shows it to be more around 18MPG. Not that 18MPG is bad for this type of vehicle but I am wondering how the computer can be THAT far off. That is a good 20% difference. I have tested this over three tanks recently and all are the same. Anyone else see this? Is this an indicator of anything else wrong?
In this day of electronic fuel control I am surprised by this. I know in other cars (I'm new to BMW) I have seen this calculated by fuel injector pulse width and pressure calcs and it is REALLY accurate. Maybe the BMW doesn't do it that specifically?

Thanks,
--Bill