According to my Edge CS my average mpg's are 13-15. My overhead display says 8-10 mpg's. I like the Edge numbers better but are they for real? I have not done my own calculations yet.

 

Neither is as good as doing the math.

 

Correct.

I have keeping track of both from the pump and the average MPG display. and found that the displayed average MPG was -1.98% of the accual using the pump data, I think less than 2% is not bad over a 9,000 mile distance.

 

I've been comparing the MPG display to my manual calculation for a couple of years now and have come to the conclusion that the overhead display is more accurate.

I say this because it is nearly impossible to fill the tank to the exact same level each time I fuel up. If the pump I'm parked at is slightly up hill or slightly down hill, it can make a difference of a gallon or two that I can pump. Yesterday I had a pump that when you just barely squeezed the lever it would immediately pump at about 1/2 capacity - no way could I trickle fuel in until it was completely full - it would just splash back out the filler neck.

So I have a lot of faith in the MPG display.