Gas Mileage Calculator

1. Fill the tank to full and note the odometer or reset the trip odometer. Drive normally until you need fuel again.
2. Fill back up to full. Enter the distance (or both odometer readings) and the fuel from the pump receipt. Pick the units that match your car and receipt.
3. Read your MPG or L/100 km. Add the fuel price for the cost, and your rated MPG to see how the car compares with its official figure.

Here is what each result means:

Gas mileage is the distance a vehicle travels on one unit of fuel. In the United States it is stated in miles per gallon (MPG). Most other countries state fuel consumption instead, in litres per 100 kilometres (L/100 km), which is the fuel used for a fixed distance. Both describe the same thing from opposite sides: a higher MPG and a lower L/100 km both mean less fuel.

Your measured gas mileage is different from the EPA rating on the window sticker. The rating comes from standard lab tests, so it is best for comparing cars. Your own figure shows what your car does on your roads, with your driving. Drivers track it to budget for fuel, to spot a fault early and to check whether a change in habits worked. Because the units mix gallons and litres, our guide to liters and gallons helps when a receipt uses one and the car uses the other.

Divide the distance since your last full tank by the fuel it took to fill up again. The fueleconomy.gov odometer method uses this exact rule: its example drove 356.2 miles (33,001.3 minus 32,645.1) on 13.5 gallons, which is 26.4 MPG. The metric and unit conversions use NIST factors:

1. Find the distance: subtract the old odometer reading from the new one, or read the trip odometer.
2. Divide that distance by the gallons (or litres) you pumped to refill to full.
3. Convert if you need another unit: 1 mile is 1.609344 km, 1 US gallon is 3.785411784 litres and 1 imperial gallon is 4.54609 litres.

To convert a pump reading on its own, the gallons to liters converter does the single step.

Trip Odometer Fill-up

You drove 300 miles and needed 10 gallons to refill. That is 300 / 10 = 30.0 MPG. In other units it is 7.84 L/100 km, 36.0 UK MPG and 12.75 km per litre. At $3.50 a gallon the tank cost $35.00, about $0.12 a mile.

Odometer Readings

The odometer read 12,000 at the last fill-up and 12,300 now, and the refill took 10 gallons. The distance is 300 miles, so the car returned 30.0 MPG.

Metric: Kilometres and Litres

You drove 500 km on 40 litres. Consumption is 100 x 40 / 500 = 8.0 L/100 km, or 12.5 km per litre. In US terms that is 235.215 / 8 = 29.4 MPG.

UK: Miles on Litres

UK odometers show miles but pumps sell litres. 400 miles on 50 litres is 400 / (50 / 4.54609) = 36.4 UK MPG. The same result is 30.3 US MPG, so the UK figure looks higher only because its gallon is bigger.

Driving Style

Fast starts, hard braking and speeding waste fuel. Fueleconomy.gov says aggressive driving can lower gas mileage by roughly 15% to 30% at highway speeds and 10% to 40% in stop-and-go traffic. On a car that returns 30.0 MPG, a 15% to 30% loss means 21.0 to 25.5 MPG.

Highway Speed

Each car has its own best speed, but fueleconomy.gov notes that gas mileage usually decreases rapidly above 50 mph. A tank of mostly fast highway driving reads lower than one at moderate speeds.

Drag and Weight

A large roof-top cargo box can cut fuel economy by around 6% to 17% on the highway, according to the same source, while rear-mounted carriers cost much less. Extra weight in the car also lowers MPG.

Trip Mix and Weather

Short trips, idling and cold weather all use more fuel per mile. One tank is a snapshot, so a winter tank of short commutes will not match a summer road trip.

How Full the Tank Is

The method only works when both fill-ups go to full. Stop 1 gallon short on the 300-mile example and the tool shows 33.3 MPG instead of 30.0. The next full tank then reads low by the same amount.

MPG and L/100 km rank cars the same way, but they show savings differently. L/100 km (like gallons per mile) grows in step with the fuel you burn, so it shows savings directly. MPG does not: the same 10 MPG gain saves far more fuel on a thirsty car than on an efficient one.

Going from 10 to 20 MPG saves 50.0 gallons every 1,000 miles. Going from 30 to 40 MPG saves only 8.3 gallons. That is why the cost side matters more than the headline MPG. At $3.50 a gallon over 12,000 miles a year, a 30 MPG car costs $1,400.00 in fuel and a 25 MPG car costs $1,680.00, a gap of $280.00.

Fuel is only one part of running a car. For insurance, repairs and depreciation, see our guide on how to calculate cost per mile.

After Every Fill-up

Logging each tank shows your normal range. A sudden drop of several MPG with no change in driving is an early sign to check tire pressure or book a service.

Planning a Road Trip

Your measured MPG gives a more realistic fuel budget than the sticker figure. Our guide on estimating a road trip's total cost adds tolls, food and lodging to the fuel.

Comparing a New Car or an EV

Knowing your real yearly fuel cost lets you compare it fairly with a newer car. For the electric side of that choice, read electric vs gas car cost.

1. Not Filling to Full Both Times

A partial fill leaves the gallons too low, so MPG reads high on that tank and low on the next.

2. Averaging MPG Figures from Several Tanks

A 300-mile tank on 10 gallons (30.0 MPG) and a 100-mile tank on 5 gallons (20.0 MPG) do not average 25.0. Add the miles and the gallons first: 400 / 15 = 26.7 MPG.

3. Mixing US and UK Gallons

A UK MPG figure is about 20% higher than the US figure for the same car. Compare like with like.

4. Using the Dash Readout as the Truth

The trip computer estimates fuel use. The pump receipt and the odometer give the measured figure.

5. Judging from One Tank

Weather, traffic and trip length move a single tank a lot. Use three or more tanks before deciding something is wrong.

6. Expecting the Sticker Figure Exactly

EPA ratings come from standard tests, so they are best for comparing cars rather than predicting your own MPG.

The arithmetic is exact for the numbers you enter; the result is only as good as the fill-up, the odometer and the receipt behind it.

Subtract the old reading from the new one and divide by the gallons you pumped. From 12,000 to 12,300 on 10 gallons is 300 / 10 = 30.0 MPG.

Divide 235.215 by the US MPG figure. 30 MPG is 7.84 L/100 km, and the same division turns L/100 km back into MPG.

The imperial gallon holds 4.54609 litres and the US gallon 3.785411784 litres, so a UK MPG figure is about 1.2 times the US one for the same fuel use.

Good means close to or better than the official rating for your own car. As a wider yardstick, EPA data put model year 2024 new vehicles at 27.2 mpg real-world on average, or 25.5 mpg without EVs and plug-in hybrids.

The sticker comes from standard tests, while your MPG reflects your speed, traffic, trip length, weather, load and maintenance. A gap of a few percent is normal.

Not reliably. The method needs a full tank at the start and the end, so the fuel you pump equals the fuel you used.

Measure at least three full tanks and add the miles and gallons together. One tank can swing a lot with weather and traffic.

Divide the price per gallon by your MPG. At $3.50 a gallon and 30 MPG, fuel costs about $0.12 a mile.

It is a weighted figure. The EPA label weights the city value by 55% and the highway value by 45%.

Yes, for any car that you fill at a pump. Plug-in hybrids also use electricity, which a fill-up does not measure.

• How to calculate your MPG (fueleconomy.gov, U.S. Department of Energy and EPA: odometer and trip odometer methods).
• Driving more efficiently (fueleconomy.gov: aggressive driving, speed and cargo effects on gas mileage).
• Learn more about the gasoline vehicle label (fueleconomy.gov: City, Highway and Combined MPG weighting).
• Highlights of the Automotive Trends Report (U.S. EPA: model year 2024 real-world fuel economy).
• NIST Guide to the SI, Appendix B.8 (NIST SP 811: mile, US and imperial gallon, and mpg to L/100 km factors).