Miles driven divided by gallons of gas.
Driving habbits that give better gas milage:
1) Highway milage vs stop n go.
2) When accelerating, don't punch it like your at a drag race.
3) Driving in Overdrive, usually the D with Box around it. Also, overdrive doesn't kick in until 45 - 50 mph.
4) Proper tire inflation
5) Change oil and filter every 3000miles.
6) Tune up
7) Winter driving, don't wait for the car to completely warm up, but don't hit the express way until you've driven at least 5mi at 50mph or less. Oil needs to warm up.
8) When standing 3min or longer, shut the engine off. Like waiting for your fast food or at a railroad crossing.
9) Use an injector cleaner in gas tank once a month. Use Techron by Chevron. The extra cost is worth it.
10) Keep a steady speed or better yet a constant engine rpm. When driving up a hill let the engine back down. By trying to maintain your speed you use more gas.