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How to Calculate gas mileage

Here is a way of calculating gas mileage or fuel-efficiency without any special tools.

Begin with your next fill-up. It is not a good suggestion to fill it right up to the top of the filler tube regarding fill-ups. This can create pressure and leakage. Instead, stop filling after the first or second time the nozzle clicks off.

Get a receipt with the liters (gallons) and the price printed or written on it. Write down the mileage of your vehicle on the receipt. That gives you all of the information you will enter into your notebook or spreadsheet in one place.

Your notebook creates columns for date, mileage, litres or gallons, total litres or gallons, total mileage, and average fuel economy.

For calculating gas mileage, you will use the two ‘total’ columns.

For metric: Divide total kilometers by 100 (= result A). Then divide total liters by result A (= result B). This gives ___ liters per 100 kilometers.

For English: Divide total miles by total gallons, which gives ___ miles per gallon.

On a spreadsheet, create the same columns and enter the appropriate formulas into the suitable cells.

Each fill or partial fill simply gets added to the total. Precise measurements and fill-ups are not overly necessary. Though this may not be precise enough for scientific calculations, it does include something that lab testing cannot duplicate:

Real-life road and driving conditions over some time.

Calculating Gas Mileage for special occasions

There may be occasions when you want specific calculations, like a trip or when you add a gas-saving device to your vehicle. You can calculate these events separately without disturbing your ongoing totals for average mileage.

  • For calculating gas mileage for a trip, you need to do only two things a little differently. Start with a fill-up at the start of the journey and end the journey with another fill-up. Record the mileage on all the receipts each time you put in gas during the trip.

After the trip, simply total the receipts from the trip separately before entering them into the notebook or spreadsheet. This will give you the mileage for your trip. If a lot of highway driving was done, you might be surprised at the improved mileage.

Of course, that will be somewhat offset by what was probably a fully-loaded vehicle, both riders, and baggage.

  • For the other scenario of adding a device, especially a fuel-saving device, to your car, I recommend filling up and then driving at least 200 kilometers or miles of your regular daily driving.

Fill up after adding the device and compare by driving another 200 km or miles, again of your regular daily driving. End the test driving by filling up again.

Drive similarly on both test drives. Remember that your drive can make a huge difference in your results, more significant than most devices will. For this reason, it is necessary to keep your driving consistent and preferably without a lot of hard acceleration and braking.

That gives you a direct comparison under day to day real-world driving conditions. Any device that claims savings of 10% or more can be confirmed with the numbers you get from the above test drive. Plus, you will notice the savings over time as your average fuel economy improves.

You may also notice that your driving changes as you focus more on calculating gas mileage. You will be less likely to race away from traffic lights. You may find it less necessary to pass everyone in front of you.

Or you’ll find yourself coasting more often as you approach slower or stopped traffic, needing minimal braking, instead of hard braking, to stop.

All these things will add significantly to your fuel economy.

Some newer cars have an average Fuel Economy calculators built-in. The car’s computer will give you an average mileage, which you can reset at any time. You can reset it for individual trips, though I prefer to use it as an overall average calculator.

I don’t reset it unless I substantially alter something that affects mileage and want to have the new fuel economy number as a base reference.

On a trip, I use the method I described above. I record mileage and keep the receipts separate until the end of the journey. You may get a pleasant surprise at your fuel economy on a highway trip if it is long enough. It is always an improvement over city driving.