MPG Calculator

How gas mileage is calculated

Fuel economy is distance divided by fuel. In the US it is quoted as miles per gallon (mpg), so you divide the miles you drove by the gallons you put back in the tank. Most of the world turns it around and quotes fuel consumption: liters used per 100 kilometers. Both describe the same thing, so one converts straight into the other.

Example: you drove 312 miles and the tank took 10.4 gallons to fill back up. That is 312 ÷ 10.4 = 30 mpg, or 235.215 ÷ 30 = 7.84 liters per 100 km. At $3.40 a gallon, each mile costs about 11.3 cents, and 100 miles cost $11.33.

The constants come from exact definitions: 1 US gallon is 3.785411784 liters, 1 imperial gallon is 4.54609 liters and 1 mile is 1.609344 km. 100 × 3.785411784 ÷ 1.609344 gives about 235.214583, and the same with the imperial gallon gives about 282.480936. A UK mpg figure is therefore about 1.2 times the US figure for the same car.

How to measure your MPG accurately: the full tank method

The dashboard readout is handy, but the most reliable number comes from the pump. The full tank method needs nothing but a few fill-ups:

1. Fill the tank until the pump clicks off the first time. Do not top it off; extra squeezes add a different amount every time.
2. Note the odometer reading, or reset a trip meter.
3. Drive as you normally do until the tank is at least half empty. The more miles between fill-ups, the smaller the effect of small differences in how full the tank gets.
4. Fill up to the first click again, ideally at the same station and pump, and note the gallons and the new odometer reading.
5. Divide the miles since the last fill-up by the gallons you just bought.

One tank can be off because of a different pump, a sloped lot or a mix of trips you do not usually make. Use the Fill-up log above to track several tanks. It averages the right way: total miles divided by total gallons. Averaging the mpg figures of each tank gives short legs too much weight. If you ever buy gas without filling to full, enter that fill-up anyway. The mpg for that one leg and the next will be off, but the average over the whole log stays correct as long as the first and last fill-ups were to full.

Why your MPG differs from the EPA sticker

The city, highway and combined ratings on a new car’s window sticker come from standardized laboratory tests on a dynamometer, with results adjusted to better reflect real roads. Every car goes through the same tests, which is what makes the numbers useful for comparing models. They cannot match every driver, and the EPA says so itself: your mileage will vary.

Common reasons your number comes in lower:

• Speed: air drag grows fast with speed. Most cars lose efficiency quickly above about 50 mph, so a lot of 75 mph highway driving costs more than the highway rating suggests.
• Short trips and cold weather: a cold engine runs rich until it warms up, and in winter that takes longer. A string of short errands is one of the hardest kinds of driving on mileage.
• Driving style: fast starts and hard braking waste the energy you just paid for.
• Load and accessories: towing, roof boxes, heavy cargo and air conditioning on hot days all take extra fuel.
• Fuel and tires: most US gas is blended with ethanol, which holds less energy per gallon than pure gasoline, and replacement tires can roll less easily than the originals.

A figure a bit below the sticker is normal. A sudden drop in a car you have tracked for a while is worth a look: check tire pressures first, then think about a dragging brake or a check-engine light.

Tips that really improve your gas mileage

• Drive smoothly. Gentle acceleration, looking ahead and coasting toward red lights instead of braking late is the biggest free improvement for most drivers.
• Slow down a little on the highway. Cruising at 65 instead of 75 mph makes a clear difference on a long drive. Cruise control helps hold a steady speed on flat roads.
• Keep tires at the right pressure. Use the number on the sticker in the driver’s door jamb, not the maximum printed on the tire, and check it monthly when the tires are cold.
• Take off roof racks and cargo boxes when you are not using them, and do not haul around weight you do not need.
• Avoid long idling. An idling engine gets zero miles per gallon, and modern engines warm up best by driving gently, not by idling.
• Combine errands into one trip, so the engine spends more of its time warm.
• Use the oil grade and fuel your manual recommends. Premium gas in a car designed for regular does not improve mileage, and devices or additives sold as gas savers have generally not held up in independent testing.

MPG vs L/100 km: why liters per 100 km is linear

Liters per 100 km measures fuel used for a fixed distance, so it is linear in fuel use: halve the number and you burn half the fuel. Miles per gallon measures distance for a fixed amount of fuel, and fuel used is 1 ÷ mpg. Equal steps in mpg therefore save very different amounts of gas. Here is what each upgrade saves over 10,000 miles:

Going from 10 to 20 mpg saves far more fuel than going from 30 to 50 mpg, even though the second jump looks bigger. When you compare two cars, compare the fuel each one uses for your distance, or look at the gallons per 100 miles figure that the US fuel economy label also shows. It works like L/100 km.

To see what a planned drive will cost with your number, enter it in the Trip Fuel Cost Calculator. To compare with an electric car, try the EV Charging Cost Calculator.

MPG to L/100 km conversion table

How to use the MPG calculator

1. 1Pick US mpg, UK mpg or L/100 km, then enter the distance from your trip meter, or your odometer readings at the start and end.
2. 2Enter the gallons or liters it took to fill up, and the gas price if you want your cost per mile.
3. 3To average several tanks, switch to Fill-up log and add each fill up. Use Convert to turn any mpg figure into liters per 100 km and back.

Frequently asked questions