How to Calculate Gas Mileage

Learn how to calculate gas mileage and take the first step to saving money on gasoline. Don’t worry, the math is easier than you might think.

| GUIDE TO GREEN CARS, Summer 2012

  • Gas Mileage Formula
    This calculation only works if you previously filled your vehicle’s gas tank and reset the trip meter to zero. After refueling, fill in the blanks to know your mpg.
  • Gas Meter
    When you refuel your car, reset your trip meter to zero. The next time you fill up, do the simple math explained in this article to determine your vehicle’s gas mileage.

  • Gas Mileage Formula
  • Gas Meter

Even if you hate doing math, it’s easy to check the real-world gas mileage of your car or truck. If you’ve avoided learning how to calculate gas mileage for fear of complex equations, fear not — anyone can do this. Plus, there are several benefits to watching your vehicle’s exact mpg.

First and foremost, knowing is half the battle. While it can be all too easy to assume your vehicle gets 30-something mpg — or whatever the sticker said when you bought the car — regularly calculating gas mileage is the best way to know how your vehicle is really performing versus what it could achieve. Furthermore, observing how mpg changes over time will give you a better understanding of positive and negative influences on gas mileage, which will make it easier to change your driving habits and thus save money on gas.

Next, think of regularly checking your vehicle’s mpg as akin to listening to it with a stethoscope. Sustained declines in gas mileage without obvious explanations (driving habits, frequent use of air conditioning, etc.) might point to a maintenance need that could become an expensive problem if left untreated.

Here are the steps to check gas mileage:

1. Fill up your vehicle’s gas tank. But know that it’s not worth topping off after the pump nozzle quits. Doing so can be bad for the environment and waste your money. Pumping in too much “extra” gas can lead to spills or even damage your vehicle’s vapor collection system, which captures harmful and polluting vapors before they can escape into the atmosphere. (Learn more in Don’t Top Off Your Gas Tank!.)

2. Reset your car’s trip meter to zero. Now you’re on the gas mileage clock.

3. Get a true test by burning at least half of your tank, then refill.

4. Write down the number of gallons it took to fill your tank and the number of miles on your trip meter. A payment receipt is an easy place to do this; it probably will already have the number of gallons printed on it.

5. Divide the trip meter miles by the gallons of gas. Bingo — that’s your gas mileage. For example: 293.1 miles ÷ 8.374 gallons = 35 mpg. (See how to calculate gas mileage with this easy formula in the Image Gallery.)

6. Reset your trip meter to zero before you drive away so you can calculate the mpg of the next tank. Start your car, and you’re back on the gas mileage clock.

If you’re a gas mileage geek like me, you’ll check mpg after every fill-up, but doing so even just once a month will give you real-world data that can help you identify trends and save money on gas. To record the numbers, you could keep a small notebook in your car or use an app on your smartphone. Even if your car keeps mpg history for you, it’s a good idea to manually check it every so often to confirm the computer’s accuracy. Think of this as a personal challenge. Can you beat last month’s average mpg? Can you beat the official fuel economy estimate for your car? For tips to maximize mpg, read How to Get Better Gas Mileage.

John Rockhold is a green car enthusiast and Contributing Editor for MOTHER EARTH NEWS. Find him on .

8/26/2013 1:38:09 PM

This is a great post on how to calculate gas mileage.

George Newell
5/16/2012 8:27:21 AM

To calculate how much additional time is required when travelling 55 mph instead of 65 mph, use the following formula: DISTANCE / MPH / 24 = #MIN_REQ. Based on this, if you are travelling 60 miles, you should give yourself an additional 10 minutes to get there at the slower speed. That means if you are travelling 178.75 miles, allow yourself an extra 30 minutes. As you can see, it really isn't much of an inconvenience to cut back your speed on the highway, but it can mean the world to you at the gas pump. I save 15% on my fuel cost. At $4.00/gal, that's like only paying $3.40 for the same gallon, and who wouldn't really like to save 60 cents per gallon?

George Newell
5/16/2012 7:36:26 AM

Calculating gas mileage (accurately) requires a slight bit more effort. Basically, when you go to refill your tank, follow these additional steps for the most accurate measurements: Go to the same gas station, use the same pump, and park in the exact same spot you did on the last fill-up. Also, fill your tank at about the same time of day and in similar weather; and of course, use the same grade/quality fuel. Something as simple as using a different pump can mean it clicks off as much as 1 gallon different than the other pumps. This will cause your mpg estimates to be incorrect; and the effect is compounded, the smaller the tank you have. As for beating the fuel estimate for my vehicle, I have definitely been able to do this. Here are some simple tricks: Keep up on maintenance (including oil changes, proper tire inflation, and even keeping the vehicle washed clean). Another big effect is your driving style. I drive a 2001 Chevy Blazer that was originally rated 15 city and 20 hwy. Around town, I average 14.5, which isn't bad for an 11-year-old vehicle. On the highway, I slow down to 55 mph and use my cruise control. I also accelerate slowly and steadily, attempting to keep the rpm's below 2000. As a result, I average 23 mpg hwy; 3 mpg OVER the original factory estimates. Travelling at 55 mph isn't any great inconvenience; I simply allow myself additional time to get where I'm going, and let all the gas guzzling idiots race past me as they race towards their destination, and end up jockeying for first place in the lineup at the next refueling stop.



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