http://www.greenhybrid.com/discuss/f...atabase-17513/

I have found mine to be off anywhere from .5mpg to 3 mpg. The link above is a little analysis I did on the database to see how much the database was skewed by people who only used the MFD to report their MPG info.

Personally I use the MFD as a guide and always calculate by hand. One thing to remember is that the hand calculations aren't 100% accurate either as you don't know if you always fill your tank up to the EXACT same point everytime.