Factory Fuel Consumption Computers
 

My 2010 Ford Focus has an info-centre with a fuel consumption computer display, how accurate are they? Can anyone point me to a link and/or give me some info on these factory computers?

...close, but not 100% accurate.

...the one on wife's Cruze is always slightly optimistic because the fuel consumption number (GALS) is consistently under-reporting at 0.95:1, so the resulting MPG value is consistently high by about 1/0.95, or about +1.053%.

...of course, this assumes (I know, I know) that the odometer-MILES numbers is correct, which it isn't exactly, but at least it's the same value being used by both the car computer and me.

so there is a margin of error on the on board computer, but it is usable. I'm hoping to get the ScanGauge some day, but with the wallet being tight, it likely wont be fore a while.

On my Impala I used to keep a paper log but after finding that the trip computer was always within +/- 1MPG of my calculations I quit.

My girlfriend had an 09 focus. The car always displayed ~2mpg higher than actual.

It will tell u relatively how well ur doing but its not perfect.

Modern on-board computers tend to be rather accurate ... in normal driving.

Mine (in what is essentially a repackaged Focus) used to be accurate down to around 3% either way before I started hypermiling. Often it was spot-on.
Hypermiling, I'm seeing deviations of up to 10%, and it usually reads low.

It won't beat a well kept fuel log ;)

As said above that it depends on several things one that the miles is correct another is how often it updates the totals and what is the max MPG it will use is it 99.9mpg or 999.9. Also the differences is based on how the fuel injectors are flowing compared to what is expected, if one is flowing more or less that the others it can calculate it wrong also if the PCM table that maps the flow of fuel expects more or less than what the average of the injectors is it will be off at that point.

Remember that the fuel injectors flow in not linear so some places the predicted flow to actual flow may be different at different loads and changes in throttle position.