well...I just filled it up and reset the trip meter and other on board travel info so Im starting my real world mileage test on this tank of gas. It has 2500 miles on it so its still pretty new. I drove it 3 miles (hardly a test) home and checked and it was showing 23.5 avg on the gauge. I'm going to do both a math calc and a gauge calc to see how close they are.

 

I have found that I have been averaging about 1.5 MPG better than the on board computer shows. Granted that I do not reset the average economy very often. I just take the miles since the last fill up and divide by the gallons it takes at the pump.