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

My mom just bought a 2009 Honda Fit with an in- dash MPG gauge.
How accurate are these displays compared to something like a Scanguage ?

From what I've read factory gauges can be as much as ~5% off.

My Insight was dead on on one trip of 650 miles. Refill calculation was exactly the same as the dash fuel mileage reading 70.2 MPG.
My Altima and the wife's Sorento both are optimistic by right at 5% as Daox posted in the above thread.
Both based on actual refill volume. If I had a scangauge I would adjust it to read as close to actual refill calculations instead of instrumented calculations.

regards
Mech

Factory mpg gauges
 

My factory Buick is very consistantly 10% optimistic but is still good at telling how I'm doing on a tankful.

If any system can consistently get within +-~3% over a wide range of driving conditions it is doing very well. Among others, things that have to be taken into consideration are fuel density, fuel pressure, system voltage and injector dead band time. All of these vary a bit, and some are not part of the equation. Mechanical positive displacement and turbine types sensors can be approach this kind of accuracy if calibrated at a specific flow rate, but the calibration has to be changed for different flow rates. Lab quality or fine process control systems can improve on those figures but it usually costs a lot of money.

Mine came pessimistic from factory, but still fairly accurate. After changing tires (new ones slightly smaller diameter), the gauge is actually more accurate. Still, only dealing with +/- 3%.

The iFCD in my MINI reads about 10% higher than the one in my ScanGauge at this point. The difference has been higher and lower in the past (different calibrations of the SG for different fill-ups), but seems to generally be about 10% higher for the car's display.

The total cumulative MPGs displayed by the car (I have not reset the total MPG readout yet) over ~3200 miles so far is 41.3 MPG. The total calculated by miles traveled/gallons filled is right at 39 MPG.

-soD

When I really want to know.....I do the fill-up vs the gps miles.
I know that my car mpg is off.....but it's always off the same. It's consistant.
When i want fixed distance mpg, I use the scangauge.

My car is about 1.8mpg optimistic or 6%.
My Ultra Gauge was 13% optimistic from the factory, but I have it dialed in now to within 1%.

the one in the escape my wife has been driving is optimistic by about 5-10% my I have dialed the mpguino down to within 1%