My friends have those MPG things in their dash too (which I think is a new dealer trend to try to change drivers habits to make the EPA mileage number seem more realistic... but anyways). But you cant trust that gauge, it's not really that accurate. The only way to truly know your true MPG is to fill up, reset odometer, drive around, fill up again and divide miles traveled by gallons needed to fill up.
Then do this every time you fill up to verify and track changes.

But hey I'm a perfectionist.