If you do a lot of gunning the throttle then rolling for long periods of time it will throw off the mpg calculation. 10 seconds of 5 mpg (during a hard acceleration) then 30 seconds of idling down the road at 40 mpg averages out to about 25 mpg. In reality you will get crummy mileage if you are constantly flooring it. Yeah these numbers are just arbitrary but I think that is about what I've seen. I think hills might also have the same effect as the heavy throttle followed by really light throttle.
Other problems would be if your tire size is too big in the programmer and it thinks you are travelling further per wheel rotation.
...That is about all I can come up with at the moment.
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