Gas Mileage (MPG) Calculator

What the gas mileage calculator does

The fuel economy on your car's window sticker is the lab's number. This tool is how you find yours. It works the way careful drivers have always worked out real mileage, from two odometer readings and the fuel you put in between them.

You give it where your odometer was at your last fill-up, where it is now, and how much gas you added, and it tells you your actual miles per gallon for that stretch of driving. Add the price per gallon and it also tells you what each mile cost you.

How to use it

Four boxes, and the price is optional.

1. Unit System. Imperial for miles and gallons, metric for kilometers and liters. The labels update so you know what to type.
2. Current Mileage. Your odometer reading right now, at this fill-up.
3. Previous Mileage. Your odometer reading at the last fill-up. The tool subtracts the two to get the distance you drove.
4. Gas Added to Tank. How much fuel you just put in to fill back up. That is the fuel that covered the distance.
5. Price per Gallon. Optional. Add it to also get your cost per mile.

Press Calculate for your mileage, or Reset to clear everything.

The fill-up method, step by step

If you have never measured your own mileage, here is the routine. It takes two trips to the pump and gives you a real number.

1. Fill the tank all the way and note your odometer reading. This is your starting point.
2. Drive normally. Do not change how you drive to chase a nice number, you want your honest everyday figure.
3. At your next fill-up, fill all the way again and note two things: your new odometer reading, and how many gallons or liters it took to fill back up.
4. Put those into the boxes above. The fuel it took to refill is exactly the fuel your driving used.

One tip that makes the number trustworthy: do this over a full tank, not a short hop, and if you can, average two or three tanks. A single short trip lets small things, like the pump clicking off at a slightly different point, throw the result off.

How the number is worked out

First the tool finds your distance by subtracting the odometer readings:

Distance = current mileage − previous mileage

Then, in imperial, your mileage is that distance divided by the gas you added:

Miles per gallon = distance ÷ gas added

In metric it reports liters per 100 km instead, which is the fuel divided by the distance, scaled to 100 km. If you entered a price, the cost per mile is your fuel spend on that tank divided by the miles you drove. Plain arithmetic, but it is your arithmetic, from your driving.

An example with real numbers

Say your odometer read 41,250 at the last fill-up and reads 41,600 now, and it took 11.5 gallons to fill back up. Gas was 3.50 a gallon.

• Distance = 41,600 − 41,250 = 350 miles
• Miles per gallon = 350 ÷ 11.5 = about 30.4 mpg
• Cost per mile = (3.50 × 11.5) ÷ 350 = about 0.12 per mile

So over that tank you got 30.4 mpg and spent about twelve cents a mile on fuel. Run the same steps next tank, and the tank after, and the average of the three is a number you can really trust.

Dashboard mpg versus the mpg you measure

Most newer cars show a fuel economy figure on the dashboard, so why measure it yourself? Because the dashboard is an estimate. It works out fuel flow electronically, and it can read a little optimistic, or lag behind your real driving. The fill-up method uses the actual fuel the pump put in and the actual distance you drove, so it is grounded in what really happened. It is common for a measured number to come out a touch lower than the dashboard's. If yours does, the measured one is usually the honest figure.

What a good number looks like

It depends entirely on the vehicle, but as a rough guide for petrol cars: around 30 mpg or more is good for an ordinary sedan, 25 or more is solid for a small SUV or crossover, and 20 or so is normal for a full size SUV or truck. Hybrids often land in the 45 to 55 range. Much under 20 for a regular passenger car is on the thirsty side. Judge your number against cars like yours, not against the whole road.

Why your mileage changes from tank to tank

If one tank reads better than the next, that is normal, not a fault. Real mileage moves with how and where you drive.

• A tank full of short, cold trips reads worse than one of steady highway miles, because a cold engine burns more.
• City traffic, with its constant stopping and starting, uses more fuel than open road cruising.
• Winter, a roof box, a loaded car, and a heavy foot all nudge the number down.
• Driving style alone makes a real difference. Research published through the US Department of Energy found that aggressive driving can lower mileage by something like 15 to 30 percent on the highway, which is exactly why averaging a few tanks gives a fairer picture than any single one.

Questions people ask

What is the most accurate way to measure my mpg?

Fill the tank fully, note the odometer, drive a normal mix of roads, then fill fully again and record the gallons and the new odometer. Use the same pump-stop habit each time, and average two or three tanks. That smooths out the small differences that one tank can hide.

Do I have to use odometer readings?

You need the distance somehow. The odometer readings give it to you by subtraction. If your car has a trip meter you reset at each fill-up, that distance works just as well, just enter your start as zero and your current reading as the trip distance.

Will premium fuel improve my mileage?

Not unless your car's manual specifically calls for premium. In an engine designed for regular, higher octane fuel gives no mileage benefit, it just costs more. Measure a tank of each if you want to see it for yourself.

Why does this differ from my dashboard reading?

The dashboard estimates fuel use electronically and can run slightly optimistic. This method uses the real fuel from the pump and the real distance, so it tends to be the more honest of the two.

References

1. National Institute of Standards and Technology (NIST), Special Publication 811: Guide for the Use of the International System of Units (SI). https://www.nist.gov/pml/special-publication-811
2. U.S. Department of Energy and U.S. Environmental Protection Agency, FuelEconomy.gov: Driving More Efficiently. https://www.fueleconomy.gov/feg/driveHabits.jsp
3. Thomas, J., Huff, S., West, B., and Chambon, P. (2017). Fuel Consumption Sensitivity of Conventional and Hybrid Electric Light-Duty Gasoline Vehicles to Driving Style. SAE International Journal of Fuels and Lubricants, 10(3). Oak Ridge National Laboratory. https://doi.org/10.4271/2017-01-9379