What your manufacturer says about your gas mileage may not be what you actually get. For example, if you regularly make short trips in your car, or if you carry a lot of cargo, your miles per gallon of gas will go down. To get your true mileage, figure the miles per gallon over a set distance of miles. The best way to do this is in between fuel stops, since you know the exact amount of gas usage between fill-ups.
Fill up your gas tank.
Write down the mileage on the odometer. You can also set the trip odometer, if one is provided on your vehicle.
Drive your car normally, as you would every day.
Fill the tank again after you have used a good portion of the fuel in the tank. The more fuel you need, the more accurate the MPG will be. If you topped off the tank on the first fill up, do that again. If you didn't top it off, don't top it off this time.
Write down the number of gallons of gas you put in the tank.
Determine the number of miles you traveled since the first fill-up, either by checking the trip odometer or subtracting your mileage at the second fill-up from that at the first.
Divide the miles traveled by the gas used. For example, if you drove 255 miles and used 10.2 gallons of gas, you would compute 255/10.2 = 25 MPG.
