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

[Old Tele man]

...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!