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How to Calculate Miles Per Gallon

A person in a black coat and jeans putting gas in their car

When you purchase your car, the window sticker provides an estimate regarding its fuel economy for both city and highway driving. Those window sticker numbers are based on best case scenario though and can vary in the real world. However, as time goes on, that fuel economy may diminish if there’s a problem with an aspect of your car’s performance. Therefore, it’s important to regularly check your car’s fuel economy to find any potential issues. If your car’s mileage is lower than usual, it may indicate that it’s in need of maintenance or repair. To determine a vehicle’s fuel efficiency, you’ll need to learn how to calculate miles per gallon.

How to Calculate Miles Per Gallon

You will want to check your car’s fuel economy every month or so. Follow these steps to calculate gas mileage:

  1. Fill up your tank. Visit the gas station, remove your car’s fuel cap and top off the tank. Starting with a full tank of gas is essential if you want an accurate result.
  2. Record your car’s current mileage. Next, you’ll need to record your car’s current odometer reading. Write down the mileage displayed on the car’s master odometer. If your car has a trip odometer, you have an easier alternative. You can simply reset the trip odometer to zero.
  3. Put some miles on your car. Drive your car as you typically would and let your fuel deplete to at least a half-tank of gas. For the most accurate measurement, let your fuel get as low as possible, but be careful not to run out of gas.
  4. Fill up again. Pay another visit to the gas station, and fill up your car’s tank. Write down the number of gallons of gas it took to fill the tank.
  5. Figure out your recent trip’s mileage. Next, you’ll need to calculate the number of miles you’ve driven since the initial fill-up. If you’re working with figures from the car’s master odometer, write down the car’s current odometer mileage and subtract it from the mileage that was on the gauge when you put gas in the tank at the beginning of this project. This will let you know how many miles you’ve driven since your first visit to the gas station. If your car has a trip odometer and you’d set it to zero at the last fill-up, you can simply record the trip odometer’s current mileage to obtain the miles driven since the initial fill-up.
  6. Calculate your car’s miles per gallon. Now that you have all of the data you need, it’s time to calculate your car’s gas mileage. Divide the miles traveled since your last fill-up by the number of gallons it took to fill the tank during your most recent stop at the gas station. The figure you wind up with is your car’s average miles per gallon.

For example here’s how to calculate mpg on a road trip. If you filled up your fuel tank and drove 200 miles, then filled up the tank again with 10 gallons of fuel, you would have averaged 20 mpg for the trip.  Now you know how many miles are in a gallon for your particular vehicle.

Leverage Technology

If you have a newer vehicle the onboard computer may answer how many miles is a gallon of fuel going to take you. Modern vehicles keep track of every step of the combustion process, from fuel flow amount to how cleanly it was burned. Through a combination of complex calculation the ECU can tell exactly how much fuel an engine is burning at any point in time. Typically this feature is part of the digital dashboard display. Look in your vehicle owner’s manual for the information display options. You should be able to select the ability to view average fuel economy and possible even instant fuel economy. Instant fuel economy tells you how many miles to the gallon your vehicle is consuming at that moment, and will vary quickly based on throttle input and vehicle speed.  This can be handy if you are trying to maximize fuel economy as it lets you modulate your throttle and speed while letting you know when you are burning the most fuel.

Use Average Miles Per Gallon to Identify Problems

A sudden drop in gas mileage may indicate a problem with your car’s performance such as a bad oxygen sensor, misfiring spark plugs, vacuum leak, or leaking fuel injectors. Your mileage may also suffer if your car has incorrect tire pressure or wheels that need an alignment. If your gas mileage suddenly drops you should set aside time for a diagnostic session or make an appointment with your local NAPA AutoCare location.

By using these steps to regularly check your car’s average miles per gallon, you’ll be provided with valuable insight into the vehicle’s performance. This may give you the information you need to identify certain performance-related issues, so you can promptly address them. Knowing how many miles in a gallon of fuel your vehicle can travel is also handy when trying to balance driving needs and fuel costs. Once you have your vehicle operating at peak efficiency, you can work on adjusting your driving habits to reduce fuel consumption.

Check out all the gas caps available on NAPAOnline or trust one of our 17,000 NAPA AutoCare locations for routine maintenance and repairs. For more information on fuel economy such as figuring the number of miles in a gallon of fuel, chat with a knowledgeable expert at your local NAPA Auto Parts store.

Photo courtesy of Unsplash.

Warren Clarke View All

I'm a writer and editor who's a regular contributor with the New York Daily News and Carfax, and my content has appeared in over 20 publications. I've written content that covers industries such as automotive, medical, insurance, healthcare, real estate, plumbing, pest control, dental and hospitality.

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