MPG from dash and Fuely are not the same
 

Just curious how this all works. I have been fuelly for a few months now, and fuelly is always a bit lower than my estimated MPG that the car displays on the dash. I might see 32.1 MPG on the dash, but Fuelly will tell me 29.0 mpg. I could see it being slightly different (32.1 vs 31.8) but it has been a bit more than slightly. What is causing this and is there something I may be doing wrong? I always fill up the car completely each time.

Thanks

I call the dash reading the optimismeter because it's usually optimistic, although better on my current car than some. I've read numerous comments about the numbers being off compared to paper/pen numbers. I think it's just the nature of the beast. Fuelly seems to match paper/pen which makes sense since it's calculated not estimated.

The dash readouts are only guides, estimates, they are never accurate 100%. Mine read consistently 3.5 MPG higher than the actual fuelly calculations, not exactly, but on average. Yours actually appears pretty accurate, some are way way optimistic.

The dash can be accurate. A calibrated Scangauge or other plug in meter is, and it is using the same data inputs as the dash display. It is just that manufacturers aren't required to have them be.

I use a ScanGauge E and my last set of mpg figures were:
Actual 52.4 mpg
ScanGauge 52.1 mpg
Onboard 53.1 mpg

However the previous top up was:
Actual 54.7 mpg
ScanGauge 52.9 mpg
Onboard 56.1 mpg

The reason the ScanGauge tends to be more accurate is down to the fact that, if necessary, you make a correction every top up. You correct the ScanGauge gallons added, to match the actual gallons added (I use half the difference as the correction factor).
For several top ups now I have not needed to make any correction as the difference has been no more than 0.1 gallons.