MPG Calculator

Quick answer: your MPG is the miles you drove since the last fill-up divided by the gallons it took to fill the tank again. 312 miles on 11.4 gallons is 27.4 MPG, which is 8.6 L/100 km. Over several fill-ups, divide the total miles by the total gallons, not the average of each tank’s MPG.

How to use it

Fill the tank until the pump clicks off and reset the trip meter, or write down the odometer. Drive as you normally do, then fill up again to the click. Type the miles on the meter and the gallons from the pump receipt: the answer updates as you type. Add a row for each fill-up after that (up to eight): the more fill-ups, the less one tank filled a little short or a little long moves the average. Switch to km and litres if your car and your receipts use them; what you typed is converted, so the answer stays the same. Optional: the price of a gallon gives the cost a mile and per 100 miles, the tank size gives the range on a full tank, and the MPG on your car’s window sticker shows how far your driving is from it. The button under the answer takes your MPG to the Road Trip Cost Calculator.

The rule, and the units

MPG is miles ÷ gallons. For several fill-ups the tool divides the total miles by the total gallons, so a long highway tank counts for more than a short city one, as it should. The other units follow from their definitions, as NIST’s guide to units (SP 811, Appendix B.8) lists them: a mile is 1.609344 km, a US gallon 3.785412 litres (3.785411784 exactly) and a UK (imperial) gallon 4.54609 litres. For litres per 100 km, NIST’s own rule is “divide 235.215 by number of miles per gallon”. A UK gallon is bigger than a US one, so the same car shows about 1.2 times more UK MPG. The cost follows fueleconomy.gov’s rule: “Cost per 100 Miles = Fuel Price ÷ Estimated MPG × 100”.

MPG in every unit

US miles per gallon against litres per 100 km, kilometres per litre and UK miles per gallon, the three units drivers outside the US use.

Worked examples

One tank before a road trip. 312 miles on 11.4 gallons is 27.4 MPG. The sticker says 31, so this car gets 12% less than its rating. With a 14-gallon tank that is 383 miles from full to empty, and at $3.50 a gallon the gas costs 12.8¢ a mile, $12.79 per 100 miles.

Two tanks, and the average that is wrong. A city tank, 180 miles on 9 gallons (20 MPG), and a highway tank, 420 miles on 12 gallons (35 MPG). The car did 600 miles on 21 gallons: 28.6 MPG. The average of 20 and 35 is 27.5, which counts the short tank as much as the long one.

A campervan in New Zealand, in litres. 520 km on 58 litres is 11.2 L/100 km, 9.0 km/L: 21.1 MPG in US gallons and 25.3 in UK ones.

Common mistakes

• Averaging each tank’s MPG. Divide the total miles by the total gallons.
• Stopping the pump at a different point each time. The gallons are what it took to refill, so fill to the first click every time, or use more fill-ups.
• Counting the first fill-up. It only starts the count: the miles are the ones driven after it, the gallons the ones at the next fill-up.
• Reading UK MPG as US MPG. A UK review’s 50 MPG is about 41.6 in US gallons.
• Expecting the sticker. In fueleconomy.gov’s words, “the EPA ratings are a useful tool for comparing the fuel economies of different vehicles but may not accurately predict the average MPG you will get.”

What a road trip does to your MPG

The tool measures what your car did. What it will do on the trip depends on how it is driven and loaded, and fueleconomy.gov (the US Department of Energy and EPA) says how much: “gas mileage usually decreases rapidly at speeds above 50 mph.” Luggage on the roof costs the most: “A large, blunt roof-top cargo box, for example, can reduce fuel economy by around 2% to 8% in city driving, 6% to 17% on the highway, and 10% to 25% at Interstate speeds (65 mph to 75 mph).” Weight matters less: “An extra 100 pounds in your vehicle could reduce your MPG by about 1%.” In the summer heat, the air conditioner on Max “can reduce MPG by roughly 5%–25% compared to not using it.” In winter, “Fuel economy tests show that, in city driving, a conventional gasoline car’s gas mileage is roughly 15% lower at 20°F than it would be at 77°F.”

Plan the drive

Pack the road trip essentials, pick a route from these American road trips you can plan in a weekend, or read up on a van life setup and winter road trips. Renting a campervan abroad? See the New Zealand road trip guide.