How accurate are are factory/OEM MPG fuel economy gauges ?

[gone-ot]

...wife's Cruze reads 5-10% too optimistic about 99% of the time, only rarely is it better.

[FXSTi]

My Fiesta can vary by +/- 2 mpg over a tank. When coasting engine off, it does not continue to keep track. My most recent fuel up (today) was 49.18 mpg calculated but the fcd showed 27ish.

Kirk

[Fat Charlie]

Mine's really accurate, except at the higher numbers it only moves in increments of 0.3. Still, as I'm going through a tank I watch the UG marching along in 0.1s and the car marching right along with it using larger steps.

My last tank was a celebration of calibration. The UG matched the pump with 14.762 gallons and almost matched the car's 424.4 miles with its 424.62 miles. The car was within 0.3 of the UG and I'm happy with that. A few tenths of a reported MPG isn't going to make me drive past a gas station when I'm almost empty. But my triputer says I should be able to go another 20 miles!

[ecomodded]

Would a long test route / test duration get more meaningful information from a mpg gauge ?
With a 5% to 10% error rate , if doing longer runs with a car for A-B-A tested would help blend the noise into real mpg figures.
Or is a 1.5km run just as adequate of a distance to gather A-B-A test information with these mpg gauges ?

[101Volts]

It depends on the gauge. I've seen a gauge in a 2000 Dodge Caravan (3.3 Litre; Not "Moony" which is tracked on this site.) which I found to be close enough to accurate at one fill-up and its instant meter also taught me that when I put the van in low gear at a certain speed, It too went into deceleration fuel-cutoff mode.

[PaleMelanesian]

Back to the original post, the 2009 Fit specifically is way off. Mine reads 12% high. There is a Service Bulletin to fix it, that I haven't had done. I have a scangauge for that.