While most modern cars will do this for you readily in their onboard trip computers, if you ever want to make sure what you’re seeing is what you’re actually getting, there’s a simple procedure by which you can determine how far your vehicle travels on each gallon of fuel it consumes.
To get an accurate picture of your fuel consumption, you’ll want to perform this following procedure a few times just to be sure your calculations are consistent. And actually, it’s a good idea to keep track of this on an ongoing basis anyway. Often an increase in fuel consumption can be indicative of another problem your vehicle has, which may have yet to fully reveal itself. By keeping track of your fuel economy on a regular basis, you can often notice problems developing before they become full-blown issues.
Fill your tank completely and take note of the mileage on your vehicle’s odometer just before you leave the fueling station. If your automobile has a resettable trip odometer, this gets even easier. Simply reset your trip odometer to zero before you leave the station.
Drive as you normally would, and continue to do so until the tank of fuel is nearly completely consumed (In other words, until your fuel gauge reading gets close to “E”). Return (ideally to the same) fueling station and completely fill the tank with the same grade of fuel you used as before. (If you bought premium, buy premium again, if you bought regular, buy regular again.) This time however, take note of how many gallons of fuel were required to fill the tank.
If you reset your trip odometer, take note of its current reading to determine how many miles you drove before you refilled the tank. If you don’t have a trip odometer, take note of the current mileage showing on your odometer and subtract the mileage reading you recorded at the prior fill-up from the current mileage reading.
The resulting number will be the observed distance your car traveled on that previous tank of fuel.
Next, divide the observed distance you calculated by the number of gallons it took to refill the tank. The result of that calculation will be the number of miles per gallon of fuel consumed your car traveled between the two fill-ups.
For example, let’s say it took 10 gallons of fuel to refill your tank and your odometer calculation shows you traveled a distance of 200 miles between fill-ups. Dividing the 200 miles driven by the 10 gallons consumed will return a result of 20 miles per gallon.
200 miles/10 gallons = 20 miles per gallon
Yeah, it’s that simple.