I make sure I fill my car up with gas. Then I clear my tripometer (or just note how many miles are on the odometer) and drive. The next time I get gas, I fill it up and I note how much gas I put in the tank, and also how many miles I drove (either by subtracting the current odometer reading from the first reading, or just getting the number off the tripometer). Then divide the number of miles you drove by how many gallons you put in your tank - you get your miles per gallon.