Know As You Go: Learn How To Calculate Gas Mileage

If you’ve ever found yourself wondering how many miles your car gets per gallon, or if you’re not sure how to calculate gas mileage, we’ve got the answers and information you need to learn more about your fuel economy. Although miles-per-gallon (mpg) calculators found online can come in handy for quick calculations, learning how to calculate mpg and fuel efficiency yourself can be a useful tool to better understand how your car is performing every time you fill up. We’ve detailed a step-by-step explanation of how to calculate miles per gallon below to help you better understand your vehicle’s efficiency.

Why Should I Calculate Fuel Economy?

Calculating miles per gallon for your vehicle can give you a more accurate and precise measurement of how your vehicle performs during your day-to-day commutes and longer trips. While some newer vehicles can tell you electronically what your current mpg rating is, and manufacturers usually provide a range of fuel economy for your specific vehicle, your location, driving preferences, and habits can have an impact on how much fuel your vehicle uses. If you’re looking for tips on how to save gas while driving, you can also check out our helpful article on how to improve fuel mileage.

How To Calculate Gas Mileage

Completely Fill Up Your Gas Tank

In order to get an accurate reading, your gas tank should be full so you can perform an assessment of the exact amount of gas your car is using. Filling your tank up halfway, or to any other level, may prevent you from measuring the exact amount of fuel your vehicle has used and could result in an inaccurate fuel economy reading. By filling your tank up until the pump automatically shuts off, you can be confident using this information to calculate the amount of gas used during your trip or between fill-ups.

A vehicle's analog odometer, with a digital display for trip meter and total mileage.

Before Moving Your Vehicle, Reset The Odometer’s Trip Meter to Zero


Most modern vehicles have a function on their odometer called a trip meter that measures short distances. The trip meter differs from an odometer, as an odometer records the total miles driven throughout a vehicle’s life, and trip meters can be reset to record miles per trip. You’ll reset the trip meter to zero since you will be using the number of miles driven with a full tank as part of the mpg equation.

You can reset the trip meter by pressing the button — usually mounted within or near the odometer and speedometer — until the trip meter reads 0.0 miles to help ensure an accurate reading during this process. In some cases, it may be necessary to go through the vehicle’s on-screen menu to reset the trip meter if it doesn’t have a dedicated reset button. If you’re unsure how to use the trip meter in your vehicle, check your owner’s manual for the correct procedure. It’s critical you reset the trip meter on your vehicle once your gas tank is full and before you’ve left the gas station to get the most accurate mileage reading.

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With The Tank Full, Drive Normally

Next, simply drive your vehicle as normal, following your daily routine that can account for common occurrences like idling, traffic, or frequent highway trips. You should also try to operate your vehicle as you normally do, since braking, air conditioning, and acceleration preferences can affect the miles your car can get per gallon. The more days you drive with this calculation in mind, the better representation you can get of your car’s average performance and fuel economy. It’s recommended you drive at least one full day before attempting the calculation for the most accurate measurement. It’s usually better, if possible, to drive for three to four days before refilling the tank and reading the trip meter since this can average daily driving variables across multiple trips.

A vehicle's analog fuel gauge with the red needle moved all the way to F for Full.

Fill Up Your Tank Again & Record The Exact Number of Gallons Used


In order to get accurate measurements needed for the miles per gallon (mpg) calculation, you will need to refill your gas tank until it is full, exactly like you did in step one. This will provide a reading that displays the number of gallons of gas you used during the previous days. It's recommended that you write this information down or get a printed receipt for peace of mind and an accurate figure.

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Check & Record Trip Meter Reading

Before leaving the gas station, access your trip odometer and determine how many miles have passed since it was reset. You can write this number down for the equation used in the next step to make sure your data is as accurate as it can be.

Divide The Trip Meter Mileage By The Gallons Used To Refill

Since we want to know how many miles per gallon your car gets, the next step is to take the two numbers you found earlier to calculate your average gas mileage. Simply divide the miles driven since you reset the trip odometer by the number of gallons used to refill your tank.

Miles On Trip Meter ➗ Gallons Used To Refill The Tank

And that’s it! As long as you ensure that your trip meter was reset before driving and that you recorded an accurate reading, this number should provide you with a good idea of your average fuel economy. This knowledge will not only help you determine the amount of fuel you might spend on a trip but can also help you notice a decline in fuel economy that could point to a larger issue with your vehicle.

Improve Fuel Economy With Parts, Tools & Accessories From O’Reilly Auto Parts

A number of factors can affect the fuel mileage in your vehicle, including where and when you drive, traffic conditions, and more. Though you have some control over how you drive your vehicle, regular maintenance can also help maintain and improve your car’s fuel economy. Regularly replacing the engine air filter and changing the oil, as well as minimizing hard acceleration and braking and running the air conditioning sparingly, may cause notable increases in gas mileage and help you save money. Whether you need a new air filter, oil change supplies, or fuel system treatment to improve your car’s gas mileage, O’Reilly Auto Parts has the right parts and supplies to perform these repairs or maintenance.

Want to know more about improving your fuel economy? You can learn how to improve gas mileage with our helpful guide, and explore the How-To Hub for a variety of repair and maintenance jobs done and save money. You can also take advantage of the O'Reilly Loaner Tool Program, which gives you access to tools you may only need once with a one-time, refundable deposit. Whether it’s advice, tools, or maintenance items, the parts professionals at O’Reilly Auto Parts are eager to help you keep your car running as efficiently as possible.

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