Well when I get a new car I always purchase a "Log" book which I keep in the glovebox. You can get them at most any major auto-parts store or you can make your own out of just a blank notepad. I track my mileage and my gas fill ups in this log annotating what my MPG is for each tank of gas. I also use the log to record tire rotations, oil changes, etc., etc., etc.
It's a great item to have when it's time to sell the car too! You can show them every oil change and what interval it was performed at along with all the other regular maintenance performed.
2016 Ecoboost Mustang
2005 Mustang V6 (Sold)
2003 Terminator (Sold)
2002 Mustang GT (Sold)