Resurrecting this thread...

In my last tank, the computer display showed 37.1 mpg but my actual calculation was 33.6. This is a large (3.5 mpg) difference. I always reset the display at each tank. This is the largest delta yet. I also checked my tires and they are at 35 PSI, so I don't see a problem there.

I have also noticed that the mpg goes up when I am using cruise control.