My 2007 Fusion (2.3 4-cylinder/5-speed manual) calculates to about one MPG plus or minus what the display shows when I fill the tank. I reset the "average MPG" display and the trip odometer when I fill the tank. Average is about 25-28 MPG in mixed driving. My commute is about 60 miles each way, 65% mountain roads, the rest on the freeway. I have seen the display as high as 49.9 MPG coming down the grade between Mammoth Lakes and Bishop, CA. I don't baby it. This car now has 107,000 miles and still drives and handles like new. No major problems.
What's fun to watch is the "instant MPG" display in my 91 Taurus (3.8) engine. It can show 99 MPG when coasting. Of course it drops to zero when you stop.