It is definitely 120 mpg. I've done it a number of times after I've filled up. The actual mpg may be higher, but the display will only go to 120. When it happens, it's actually kind of funny--you can hit the gas hard, knowing that you're lowering the mpgs, and it will take about 30 seconds before it'll actually start coming down, showing that it was actually way above 120. Of course, this has to be just after you've filled up, on a downhill slope....