I've never been able to definitively measure our fuel mileage, due to a couple of reasons. Our tank is very slow to fill and we're never quite sure if it's full because after waiting a while for it to drain down we can usually add an extra 7-10 gallons. We changed the rear gears and the odometer is no longer accurate.
But by using a GPS and waiting a long time for the tank to drain down so we could get it as full as possible, I discovered that we're getting 13-14 MPG on flat ground!!!!
I thought that couldn't possibly be right, but even adding an error allowance of 50 miles it's still close to 13 mpg! Here we thought we were getting 10 mpg all along.
Now I wonder what we get going over the Rockies...