Fuel Efficiency Converter

The fuel efficiency converter switches fuel consumption figures between litres per 100 kilometres (L/100km), miles per US gallon (mpg), miles per imperial gallon, and kilometres per litre (km/L). You enter one number, pick the unit it is in, and read the same consumption expressed in the other units. The tool is useful for anyone comparing vehicles across markets, reading a European fuel figure as an American one, checking a rental car's real consumption, or working out what a car's official figure means in money terms.

Fuel efficiency has two common conventions that point in opposite directions. Europe, Africa, Asia, and Australia quote litres per 100 kilometres, where a smaller number means better economy. The United States, the United Kingdom, and Canada quote miles per gallon, where a larger number means better economy. A car doing 5 L/100km is economical. A car doing 50 mpg is economical. Both statements describe the same kind of car, so converting between the two systems is a routine task for buyers, fleet managers, and journalists.

Why the US and UK gallons differ

The single most common source of error in fuel economy conversion is the gallon. The US gallon and the UK (imperial) gallon are different sizes, so "mpg" is not one unit but two.

A US gallon is exactly 3.785411784 litres. An imperial gallon is exactly 4.54609 litres, about 20.1% larger. The difference traces to the two systems' separate histories: the imperial gallon is defined as the volume of 10 pounds of water at a stated density, while the US gallon descends from the Queen Anne wine gallon of 231 cubic inches.

The practical consequence: the same fuel consumption produces a higher mpg figure in the UK system than in the US system. A car that does 30 mpg on the US gallon does about 36 mpg on the imperial gallon, even though it burns the same amount of fuel. Any converter, this one included, must ask which gallon you mean, and any article quoting "mpg" without saying which gallon is incomplete.

The conversion formulas

All the conversions on this page are built on two exact definitions: one mile equals 1.609344 kilometres, and the two gallon sizes are fixed constants. From those, the conversion factors between L/100km and mpg follow directly.

To convert L/100km to US mpg:

mpg (US) = 235.214583 ÷ L/100km

To convert L/100km to imperial mpg:

mpg (UK) = 282.480936 ÷ L/100km

To convert to kilometres per litre:

km/L = 100 ÷ L/100km

The constants 235.214583 and 282.480936 come from combining the two exact definitions. The first is 100 km in miles (62.1371192) divided by litres per US gallon (0.2641720524). The second uses the imperial gallon in place of the US gallon. Because the underlying definitions are exact, the conversion is exact; the only rounding is in how many digits you choose to display.

To reverse the direction, divide instead of multiply: L/100km = 235.214583 ÷ mpg (US), or L/100km = 282.480936 ÷ mpg (UK).

Worked examples

Example 1: a typical European hatchback at 6.5 L/100km.

Using the US formula: 235.214583 ÷ 6.5 = 36.2 mpg (US). Using the imperial formula: 282.480936 ÷ 6.5 = 43.5 mpg (UK). The same car looks 20% more efficient in a UK advertisement than in a US one, purely because of the gallon definition.

Example 2: an American pickup at 20 mpg (US).

Reverse the US formula: 235.214583 ÷ 20 = 11.8 L/100km. In kilometres per litre: 100 ÷ 11.8 = 8.5 km/L. A truck that looks reasonable in US terms (20 mpg) actually uses nearly twice the fuel of the hatchback in example 1.

Example 3: a UK-market diesel at 55 mpg (imperial).

The UK figure uses the imperial gallon: 282.480936 ÷ 55 = 5.1 L/100km. If someone converted that 55 mpg as though it were US gallons, they would get 235.214583 ÷ 55 = 4.3 L/100km, which understates real consumption by about 16%. Mixing up the gallons makes the car look better than it is.

Example 4: a small SUV at 9 L/100km on a mixed route.

US mpg = 235.214583 ÷ 9 = 26.1. UK mpg = 282.480936 ÷ 9 = 31.4. km/L = 11.1. These are the three numbers a South African buyer comparing against imported US and UK reviews would want side by side.

How to measure your real fuel consumption

The official figure on a window sticker comes from a laboratory cycle and almost never matches real driving. To measure your own car's true consumption, use the brim-to-brim method:

Fill the tank completely and reset the trip odometer, or note the current odometer reading.
Drive normally until the tank is at least half empty. Longer is better; a full tank of driving gives a more reliable average.
Fill the tank completely again at the same pump, if possible. Note the litres shown on the pump and the distance driven.
Divide litres by kilometres driven, then multiply by 100 to get L/100km. For example, 42 litres over 620 km gives 42 ÷ 620 × 100 = 6.77 L/100km.
Repeat across a few tankfuls. A single tank can be distorted by traffic, weather, load, and tyre pressure; the average of several is the number worth quoting.

For a US or UK mpg figure, convert the measured L/100km with the formulas above, using the gallon that applies where you drive.

Fuel cost implications

Fuel efficiency matters because fuel is a running cost that repeats. The same consumption difference that looks small per tank adds up over a year. The arithmetic is simple:

cost per 100 km = L/100km × price per litre

annual cost = (annual km ÷ 100) × L/100km × price per litre

A concrete pair of figures makes the point. Suppose fuel costs 22 rand per litre and you drive 20,000 km a year. A car at 8 L/100km costs 8 × 22 = 176 rand per 100 km, and 200 × 176 = 35,200 rand per year. A car at 6 L/100km costs 132 rand per 100 km and 26,400 rand per year. The 2 L/100km difference is worth 8,800 rand a year, which is real money over a three-year ownership period.

The same maths works in any currency. In the United States at 3.5 dollars per US gallon, a car doing 30 mpg uses 20,000 miles ÷ 30 = 667 gallons a year, costing about 2,333 dollars. At 22 mpg the same mileage uses 909 gallons and costs about 3,182 dollars. The gap of roughly 850 dollars a year is the price of choosing the less efficient option.

Two cautions about cost estimates. First, the price per litre you pay is a local number that changes constantly, so treat any annual figure as an estimate, not a prediction. Second, real-world consumption is usually higher than the official test figure, sometimes by 10 to 20%, so budgeting on the official number is optimistic.

Common mistakes to avoid

Using the wrong gallon. This is the big one. Check whether the mpg figure is US or imperial before you convert. A UK review quoting 60 mpg is not the same efficiency as a US figure of 60 mpg.
Inverting the division. L/100km and mpg move in opposite directions. A smaller L/100km is a bigger mpg. Dividing when you should multiply produces an answer that is wrong by roughly 235 (or 282) times.
Converting distance and volume separately and getting the units tangled. The constants already fold the mile and the gallon in; do not also multiply by 1.609344 or divide by 3.785.
Trusting a single tankful. Short measurement runs are noisy. One tank with a traffic jam in it does not represent the car.
Comparing official figures across different test cycles. A US EPA figure, a European WLTP figure, and an old NEDC figure each come from different laboratory conditions; converting the units does not reconcile the test methods.

Limitations and assumptions

The converter assumes the standard definitions above: the international mile of 1.609344 km, the US gallon of 231 cubic inches, and the imperial gallon of 4.54609 litres. It does not account for fuel grade, driving style, terrain, traffic, load, or weather, all of which change real consumption. It converts efficiency figures; it does not measure your car. For that, use the brim-to-brim method.

The tool is not a fuel price service and does not claim to predict what you will pay at the pump. Cost examples in this guide use illustrative prices only.

Frequently Asked Questions

How do I convert L/100km to mpg?

Divide 235.214583 by the L/100km figure for US gallons, or divide 282.480936 by the L/100km figure for imperial gallons. For example, 7 L/100km is 33.6 mpg (US) or 40.4 mpg (UK).

What is the difference between US and UK mpg?

The US gallon is 3.785411784 litres; the imperial gallon is 4.54609 litres. Because the imperial gallon is about 20.1% larger, the same fuel consumption yields an mpg figure about 20% higher when expressed in UK units. Always confirm which gallon a figure uses.

Which is better, lower L/100km or higher mpg?

Lower L/100km is better; higher mpg is better. The two scales run in opposite directions. A car at 5 L/100km (about 47 mpg US) is more efficient than one at 8 L/100km (about 29 mpg US).

How many litres does it take to drive 100 km at 40 mpg?

At 40 mpg (US): 235.214583 ÷ 40 = 5.88 L/100km. At 40 mpg (UK): 282.480936 ÷ 40 = 7.06 L/100km. The imperial figure is the worse fuel consumption because the same mpg number means more litres per 100 km in UK units.

How do I measure my car's real L/100km?

Use the brim-to-brim method: fill the tank, drive, refill to the same point, then divide litres refilled by kilometres driven and multiply by 100. Average several tankfuls for a reliable figure.

Is the official fuel consumption figure accurate?

No. Laboratory test cycles such as WLTP or EPA are run in controlled conditions and typically understate real-world consumption by 10 to 20%. Use them for comparing cars, not for budgeting fuel spend.

How much does 1 L/100km difference cost per year?

At 22 rand per litre and 20,000 km a year, 1 L/100km is worth 200 × 1 × 22 = 4,400 rand a year. At 3.5 dollars per US gallon and 20,000 miles a year, 1 mpg at the 30 mpg point is worth roughly 90 dollars a year.

Why do Canadian and US mpg figures differ?

Canada moved to litres per 100 km for official ratings, but many Canadian sources still quote mpg using the US gallon, which is the standard gallon for fuel in Canada. Canadian mpg figures are normally US mpg, not imperial.

References

NIST Special Publication 811, Guide for the Use of the International System of Units (SI), for the exact definition of the US gallon in cubic inches and the litre.
The Weights and Measures Act (United Kingdom), which fixes the imperial gallon at 4.54609 litres.
ISO 80000-1, Quantities and units, for the kilometre and the litre as SI-derived units.
The International Yard and Pound Agreement of 1959, which fixes the mile at 1.609344 km.

Q: Can the Fuel Efficiency Converter: L/100km to MPG be used for professional or commercial purposes? A: Yes, the Fuel Efficiency Converter: L/100km to MPG provides mathematically correct results that are suitable for professional, commercial, and educational use. For the Fuel Efficiency Converter: L/100km to MPG, For the Fuel Efficiency Converter: L/100km to MPG, For high-stakes applications (medical, legal, financial), verify results with a domain expert. For the Fuel Efficiency Converter: L/100km to MPG, the Fuel Efficiency Converter: L/100km to MPG formulas used are well-established and validated against reference standards.
Q: How often are the Fuel Efficiency Converter: L/100km to MPG formulas updated? A: the Fuel Efficiency Converter: L/100km to MPG formulas are based on established scientific, mathematical, or industry-standard references and rarely require updates. When standards change (e.g., new physical constants, revised tax brackets, updated standards), the Fuel Efficiency Converter: L/100km to MPG is updated to reflect the current authoritative source. For the Fuel Efficiency Converter: L/100km to MPG, For the Fuel Efficiency Converter: L/100km to MPG, Each calculator's references section lists the specific sources used.

Fuel Efficiency Converter