There are two basic ways to calculate gas mileage. The task is quite simple if your car, truck, or SUV has an Average Fuel Economy gauge – which may also be referred to as an Average MPG (Mile Per Gallon) display. If it’s not so equipped, there’s no need to worry … the task is not difficult.

If your car has a built-in average fuel economy display, upon filling your vehicle’s gas tank:

- Write down the number on the Average Fuel Economy gauge.
- Reset the Average Fuel Economy gauge.
- Write down the odometer reading, for good measure.

As you’re driving, take the time to check the Average MPG display. You’ll notice that this figure will fluctuate over time, as you encounter different driving conditions.

The next time you fill up your tank, take note of both the odometer reading and the final number on the Average Fuel Economy display. Write both of these figures down, along with the number of gallons it took to refill your fuel tank.

If you keep a log book, make note of any special conditions you encountered that week, such as inclement weather, heavy traffic, or a different daily routine.

If your car is not equipped with an Average MPG display, each time you fill up:

- Write down the odometer reading and the number of gallons, then
- Subtract the new odometer reading from the previous odometer reading, and
- Divide that number by the number of gallons.

So for example, if you started with 21023 miles on the odometer and ended with 21335, you would have traveled 312 miles (21335 minus 21023). If you put 11.3 gallons in the tank, you’d divide 312 by 11.3 … resulting in 27.6 miles per gallon.

*Tip: Remember to reset your Trip Meter each time you fill up. Doing so will provide the number of miles traveled, without the need for subtraction.*

There will always be slight variations from tank-to-tank. If you’re using an Average Fuel Economy gauge, you may want to double-check it against the hand calculation.

There are two tools on MPGomatic that can help you see the bigger picture.

Our Gas Mileage Calculator lets you compare the fuel efficiency of one vehicle against another, allowing for differences in both fuel economy and the price of fuel (if you want to compare regular unleaded to premium or E85, for example).

The What’s My MPG Calculator does the basic math as describe above. All you need to do is input the number of miles traveled, the number of gallons, the cost per gallon, and the number of days between fill-ups.

So using the same numbers from before, while adding the cost of gas as $3.50 and the seven days between stops at the service station, we learn that your car is not only getting 27.6 MPG, but that you’re using an average of 1.614 gallons per day. At that rate of consumption, you will use approximately 589.2 gallons in a year at a cost of $2062.25.

*– by Daniel Gray*

As gas prices rise, fuel efficiency is becoming more and more of a critical factor. Knowing your car’s MPG can help you determine if it’s a gas guzzler that’s eating up your wallet as well. Once you figure out the MPG, you can do many useful things, like calculate how much a 10¢ rise in gas prices will affect your budget, or how getting a car with better MPG will lower your monthly costs.