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.

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.


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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!.)