My 2009 Fit had an issue of calculating MPG as well. It would overestimate the MPG by about 6 MPG every tank. About 8 months after I got it, Honda issued a software update to fix it. The dealer had no clue about it, but I showed them the Technical Service Bulletin from Honda, and they ran the update while it was in for an oil change. After that, the MPG indicator has been right on, always within about .2 of my calculated average.
So it seems like there may definitely a problem with the MPG math, but hopefully after Ford has had some time they can issue an update to fix the issue.
Sterling Grey 2012 Hatchback Titanium with all factory-installed options born 4/27, MFT 2.11 (usb install), transmission updated 1/12/11
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