Gas mileage is calculated by dividing the number of miles traveled by the number of gallons of gasoline it took to travel those miles. To get an accurate figure, reset the car's trip meter to zero immediately after filling the car up, then do the calculation at the next refill.
Monitoring a car's gasoline mileage on a regular basis is important as a drop in gasoline mileage can indicate a problem. In many cases, simple fixes such as inflating tires to the correct pressure or replacing a clogged air filter will improve gasoline mileage. Improving poor driving habits and removing extra weight from the trunk or cargo area will also improve gasoline mileage.