How does it calculate MPG?

 

You callibrate it over several tanks of gas. You callibrate both how much gas it actually takes to fill your Disco as well as calculating what actual distance traveled is (vs what the odometer reads - ie. I actually travel about 1.2 miles for every mile on the odometer) It will measure the distance you travel and track how many gallons it took to travel that distance. It has proved to be pretty accurate...