I directly calculate the MPG using a full tank, reset the trip meter, and then fill the tank again (preferably at the same station and pump) and then use the gallons that go in to figure the MPG.
I find that the trip computer lies. Best I've ever gotten in the winter was just pushing 15 MPG on level interstate going 60MPH. I'm going to take a closer look this summer at the warm MPG. But if you're going downhill, with a tailwind, you'll probably get upwards of 17MPG.
A programmer would definately help!