I did this for a while by using a simple stop watch that I paid $5 for at a local sporting goods store. Basically, when I started the car I also started the stop watch, when I turned off the car I stopped the watch. I was using it to track engine hours for maintenance and to validate my current presumption that my driving style works out to about 4,000 miles per 100 engine hours. I use 100 engine hours as my basic oil change interval since it is much more indicitave than mileage on your odometer to the life of the oil. If you're looking for long term data that isn't a very good option, for me I just used it about5 tanks to validate my theory of average mph per engine hour. It is a simple, low cost, low technology way to give you a pretty close idea on your driving habits assuming your commute or driving style doesn't change significantly from week to week.