Fuel Economy Converter - mpg to L/100 km & km/L

About the Fuel Economy Converter

Car fuel economy is stated in three different ways around the world, and they do not convert by simple multiplication. Americans quote miles per US gallon, the British miles per imperial gallon, and most of the rest of the world litres per 100 kilometres; some countries also use kilometres per litre. Comparing a US car review with a European brochure, or a British road test with an Australian one, therefore needs a proper conversion.

This fuel economy converter converts between US mpg, UK mpg, L/100 km and km/L. It shows the figure in all four units, how much fuel the car uses for 100 km and 100 miles, and a simple rating from very economical to very thirsty.

How to Use the Fuel Economy Converter

Enter the fuel economy figure.

Choose the unit it is in, from, and the unit you want, to.

Check which gallon a figure uses: American sources mean US gallons, British sources imperial gallons.

How Fuel Economy Conversion Works

Miles per gallon and kilometres per litre measure distance per fuel. Litres per 100 km measures fuel per distance. Converting between the two kinds means taking a reciprocal, so the converter divides a constant by the figure:

  L/100 km  = 235.215 ÷ US mpg
  L/100 km  = 282.481 ÷ UK mpg
  L/100 km  = 100 ÷ km/L

  UK mpg    = US mpg × 1.20095    (the imperial gallon is larger)

The constants come from exact definitions: 235.215 is 100 km × 3.785411784 litres per US gallon ÷ 1.609344 km per mile.

Step-by-Step Example

30 US mpg to L/100 km.

  235.215 ÷ 30 = 7.84049 L/100 km

A car doing 30 US mpg uses about 7.84 litres per 100 km. The same car is 36.0285 UK mpg and 12.7543 km/L. Over 100 miles it burns about 3.33 US gallons, or 12.6 litres.

5 L/100 km to mpg is 235.215 ÷ 5 = 47.0429 US mpg, or 56.4962 UK mpg — the kind of figure a small hybrid might achieve.

50 UK mpg is only about 41.6337 US mpg, because the British gallon is larger.

Why the Scales Behave Differently

Because L/100 km is a reciprocal of mpg, equal steps on one scale are not equal on the other. Going from 10 to 20 mpg saves far more fuel than going from 40 to 50 mpg. Over 10,000 miles, the first change saves 500 US gallons; the second only 50. In L/100 km the difference is obvious: 23.5 to 11.8 L/100 km, against 5.88 to 4.70. This is why many economists argue that fuel-per-distance measures such as L/100 km, or gallons per 100 miles, give a clearer picture when comparing cars.

Typical Figures

  very economical   under about 4 L/100 km   (over about 59 US mpg)
  economical        4 – 6 L/100 km           (39 – 59 US mpg)
  average car       6 – 9 L/100 km           (26 – 39 US mpg)
  thirsty           9 – 13 L/100 km          (18 – 26 US mpg)
  very thirsty      over 13 L/100 km         (under 18 US mpg)

Official test figures are usually better than real-world driving, which includes traffic, cold starts, air conditioning, roof racks and motorway speeds. Allow 10 to 25 percent more fuel than the published number for everyday use.

Electric and Hybrid Cars

Electric cars are rated in energy per distance, such as kWh per 100 km or miles per kWh, and US labels also give MPGe, miles per gallon equivalent, which treats 33.7 kWh of electricity as one gallon of petrol. Plug-in hybrids have two figures, one for each power source. This converter handles liquid fuel figures; compare electric efficiency in kWh.

From Economy to Cost

Fuel economy becomes most useful when turned into money. For a car doing 7.84 L/100 km, a 400 km journey needs about 31.4 litres; at a price of 1.80 per litre that is about 56.45 in fuel. In US units, the same car at 30 mpg needs 10 gallons for 300 miles, costing 35 at 3.50 a gallon. Converting the economy figure into the unit your fuel is priced in — litres or gallons — keeps the arithmetic simple, which is why the converter shows fuel for 100 km and 100 miles in both. For a whole year of driving, multiply by the annual distance to see how much a more economical car would save.

Why Economy Changes With Speed

Most cars are at their most economical at steady speeds of roughly 50 to 90 km/h, 30 to 55 mph. Above that, air resistance rises steeply, so motorway driving at 130 km/h can use a quarter more fuel than at 100 km/h. Short trips from cold, stop-start traffic, heavy loads and under-inflated tyres all make the figure worse.

Understanding Your Result

The headline is the converted figure.

The in every unit line gives the economy in all four units.

The fuel for a trip line shows litres per 100 km and fuel for 100 miles in US gallons and litres.

The rating places the figure on a simple scale from very economical to very thirsty.

When Should You Use This Calculator?

Use it to compare cars reviewed in different countries.

Use it when buying an imported car or reading a foreign brochure.

Use it to understand the fuel use of hire cars abroad.

Use it with fuel cost and trip calculators that need a particular unit.

Common Mistakes

Multiplying instead of dividing. mpg and L/100 km are reciprocals; use the constants.

Mixing US and UK gallons. UK mpg figures are about 20 percent higher for the same car.

Thinking a lower L/100 km is worse. For L/100 km, lower is better.

Comparing mpg improvements by size alone. Gains at low mpg save much more fuel.

Trusting official figures fully. Real driving usually uses more fuel.