A Free Calculator That Calculates How Much Money You Can Save By Getting Better Gas Mileage! First it calculate your MPG and then compares the cost per mile you are paying with the cost of other MPG's. It is simple to use... Write down the number of miles on the odometer at fillup. Then write down how many gallons to the thousandths (.xxx) it took to fill the tank. Finally write down the miles on the odometer when the gas guage gets to the same place it was when you made the previous fillup. Enter these numbers in the calculator. Note: if you need help figuring when you have used the previous fllup's gas, put a piece of masking tape above (or bellow) the gas guage and put a pencil mark at the spot the needle was at the first fillup of the test.