I use Excel spreadsheet to track mileage on all of my vehicles, a tab for each. You can get apps that will do that too. I keep track of miles, quantity, total ODO reading, what station, brand, location, etc and it figures the cost per mile also. I can tell you if one station gives me unusually bad or good mileage over other stations or gas.
Sounds OCD but I started it in Oct of '11 when a new to me car that I bought was particularity difficult to fill up consistently from one fillup to the next. I can see the distinct difference between summer blend and winter blend gasoline and tell you within a week when they switched.
It is now something that I just do, and its kinda neat to see what the car will actually do.
Charles