Gas Mileage (MPG) Calculator

MPG for common miles and gallons

Miles per gallon is one division: miles driven divided by gallons used. Drive 350 miles on 14 US gallons and that is 350 divided by 14, which is exactly 25 MPG. Any pair of numbers works in the calculator above; these are the fill-ups people look up most.

MPG converted to litres per 100 km, km per litre and UK MPG

A US gallon is 3.78541 litres and a mile is 1.609344 km, so litres per 100 km is 235.214 divided by your MPG. That is a reciprocal, not a scale: lower litres per 100 km is better, while higher MPG is better. A UK (imperial) gallon is 4.54609 litres, about 1.201 times a US gallon, so the same car always shows a higher MPG number in UK figures than in US ones. MPG measures efficiency rather than money, so to turn one of these figures into the cost of a journey you also need the pump price: the fuel cost calculator does that part.

Rounded for display. Figures are calculated from the exact conversions (3.78541 litres per US gallon, 4.54609 litres per UK gallon, 1.609344 km per mile), so a UK MPG figure quoted for a car is not directly comparable with a US one.

Litres per 100 km converted back to MPG

European spec sheets quote consumption the other way round, as litres burned per 100 km, and the conversion is its own inverse: divide 235.214 by the litres figure and you get US MPG back. A car listed at 6 litres per 100 km is about 39.2 US MPG, or 47.1 UK MPG. Watch which gallon a quoted figure uses before comparing two cars: a UK MPG figure of 47 and a US one of 39 can describe exactly the same car.

Going the other way from a UK figure, divide by 1.201 to get the US equivalent: 50 UK MPG is about 41.6 US MPG.

How combined MPG is calculated from city and highway figures

A US window sticker quotes three numbers, and the combined figure is not the average of the other two. The EPA weights city driving at 55 percent and highway at 45 percent, but it applies those shares to fuel burned per mile rather than to the MPG figures themselves, then inverts the total back into MPG. In full: divide 0.55 by the city figure, divide 0.45 by the highway figure, add them, and take one divided by the result. A car rated 24 city and 34 highway lands at 27.7 MPG combined, below the 29.0 an average would give.

The gap is not a rounding artefact. Averaging MPG treats a mile driven badly and a mile driven well as equal, when the thirsty mile actually burns more of the tank, so the combined figure is always pulled toward the lower of the two numbers. The wider the two figures are apart, the bigger the error: the last column below is how much a simple average would overstate the car.

The last row is a hybrid, the one common case where the city figure beats the highway one, because stop-start traffic is where regenerative braking and the electric motor do their work. It is also the only row where the average comes out low rather than high: the 55percent weight sits on the car's stronger figure, which more than offsets the harmonic penalty.

The same rule applies to your own fill-ups

If you have logged two tanks, do not average the two MPG readings. Add the miles, add the gallons, and divide once. A 100 mile crawl at 15 MPG uses 6.67 gallons and a 100 mile motorway run at 40 MPG uses 2.50, so the 200 miles cost 9.17 gallons in total. That is 21.8 MPG, not the 27.5 an average of the readings suggests. The calculator at the top of this page does exactly this if you enter the totals rather than one tank at a time.

How many gallons a trip needs at a given MPG

Once you know your MPG, the fuel a journey needs is the same division run backwards: distance divided by MPG. A 300 mile drive at 25 MPG takes 12.0 US gallons, and at 40 MPG it takes 7.5. Treat these as a floor rather than a plan, because motorway speed, hills, cold starts, roof boxes and a full car all push real consumption above the rated figure.

Gallons here are US gallons. Multiply by 3.78541 for litres, so the 300 mile trip at 25 MPG is about 45.4 litres.