1992 300E, 173k mi.

Best: 26.4
Worst: 19.82

Average: 22.46

And I keep track using my handy Zaurus handheld, with an mpg calc I downloaded. Keeps track of the date, the mileage, mpg, and total number of gallons.

The only other feature I'd like is location and price, so I could see more easily what gas station gives me the best mileage for the price (I'm going to have to remember to email the author about that...).