Record your MPGs

A basic excel sheet is all you need to keep track of your vehicles fuel economy. The eaiest way to do this would be to record your vechels total mileage at every fillup.  While also recording the number of gallons used for every tank.
 
If you only put a few gallons at a time in your gas tank then your costing your self more money.  It's better to buy your gas by the tank when it's cool out if possiable.