If I may interject, for those wishing to track their mileage, there is something they should always do when filling up:
- Get a printed receipt (contains the amount of fuel added and price paid)
- Write on that receipt your current odometer reading (not your trip meter)
Mileage is just the distance travelled compared to fuel used between consecutive fills. But doing the math is tedium. You could build or buy your own spreadsheet/database, but there is a fabulous online resource that will do all the work for you:
http://www.spritmonitor.de The site is German-native, and some bits of it haven't been localized for English yet, but it's not hard to use. TIP: Look for the British flag in the top-right corner. Set up a personal account profile, create vehicle profiles, and start tracking with the receipts you're collecting from the gas station. You can also indicate driving style, type of roadway used, tire type, and grade of fuel. With all that information it can give you some meaningful analysis. Best of all, it provides a way to compare your results with other people's vehicles of the same type or class, and you can get your own mileage reading to include in your signature (like what you see in this post, below).