Two things:
* Make sure that high speed data reporting is on. THe MPG calculations suffer badly if that is not turned on.
* Once it IS turned on, measure you r MPG at the pump and compare it to the app , there is a setting for MPG calculation compensation. I currently have mine set to 1.03
My readings were insanely high (like 27 mpg) before turning on the high speed data mode. Then they were a bit too low so set the compensation value.