I always reset my MPG and average speed at every refill. That way, I can accurately calculate the MPG and compare it to the computer, plus the average speed gives me a good measure of my city/highway mix. I can calculate my long term stats by keeping a log, if I desire. Doing it this way, I easily note the improvement with break-in, as well as the decline with winter driving, and subsequent improvement in the spring. My old BMWs had two mpg computers, so I could reset it for each trip and each tankful, as well as an instantaneous display; I really liked those features. Using this method, one will see some impressive numbers at times, like after being on the highway for 10+ miles right after a fill-up. You can also see some really bad numbers in a traffic jam, or just watching what idling does. You can teach yourself how to drive more efficiently if you desire, such as during gas crises or times of high gas prices, by watching the average mpg with each change in habits or style.