You can convert fuel economy online in seconds with the free Fuel Economy Converter: enter a figure, choose its unit and the unit you need, and 5 litres per 100 km becomes 20 kilometres per litre, 47.0 MPG (US) and 56.5 MPG (UK) at once. The tool works with four units — litres per 100 km, kilometres per litre, MPG (US) and MPG (UK) — and this guide gives the exact formulas behind each one, including why these conversions divide instead of multiply.
Fuel economy is the one conversion in this series where the numbers run backwards. In most unit families a bigger number means more of the thing: more watts, more joules, more degrees. Here, one of the major units measures the opposite quantity from the other three. Litres per 100 km counts fuel burned over a set distance, so a bigger figure means a thirstier car. Miles per gallon and kilometres per litre count distance achieved on a set amount of fuel, so a bigger figure means a thriftier car. Put a European consumption figure beside an American economy figure without converting, and you will rank the cars in exactly the wrong order. Add the fact that the American and British gallons are different sizes — so the same car earns two different MPG numbers — and this becomes the conversion most worth doing slowly, with the formulas visible.
How Do You Convert Fuel Economy Online, Step by Step?
- Open the converter. Go to the Fuel Economy Converter on Asad Webs.
- Enter the figure exactly as published. Copy it from the brochure, listing, dashboard or review, with its unit. The unit tells you whether the number is a consumption figure or an economy figure, which decides its direction.
- Select the unit you have. Choose litres per 100 km, kilometres per litre, MPG (US) or MPG (UK) — and if the source just says MPG, establish which gallon it means before you choose. American sources mean US gallons; British sources mean UK gallons.
- Select the unit you need. Pick the unit used by the other car, the other market or your own fuel records, so the comparison that follows is like with like.
- Apply the direction check. Converting from L/100 km to any other unit must flip the ranking sense: the car with the higher L/100 km figure must come out with the lower MPG and km/L figures. If both numbers rose together, something has been swapped.
- Record the unit with the result, every time. An unlabelled 47.0 could be a good MPG (US) figure or an impossible L/100 km figure. The label is not decoration here; it is half the information.
The Four Units and the Formulas Behind Them
Two ideas generate all four units. Consumption asks: how much fuel does a fixed distance need? Litres per 100 km answers it directly, which is why engineers and European regulators favour it — it scales honestly, so doubling it really does mean double the fuel for the same trip. Economy asks the reverse: how far does a fixed amount of fuel go? Kilometres per litre and both MPG units answer that. Because the two ideas are reciprocals of each other, every conversion between the families is a division into a constant, not a multiplication by one — the mathematical signature of this whole topic, and the reason the formulas below look different from every other guide in this series.
| Unit | What it counts | Formula from L/100 km | Higher number means |
|---|---|---|---|
| Litres per 100 km (L/100 km) | Fuel used over 100 km | The starting unit itself | Worse — more fuel burned |
| Kilometres per litre (km/L) | Distance from one litre | km/L = 100 / (L/100 km) | Better — more distance per litre |
| MPG (US) | Miles from one US gallon | MPG (US) = 235.215 / (L/100 km) | Better — more miles per gallon |
| MPG (UK) | Miles from one UK gallon | MPG (UK) = 282.481 / (L/100 km) | Better — more miles per gallon |
The two constants are not arbitrary. Each one bundles the mile-to-kilometre relationship together with the size of its gallon, folded into a single number to divide by. The UK constant is larger — 282.481 against 235.215 — purely because the UK gallon is larger, so a car goes further on one of them. That is also why a British review and an American review of the identical car print different MPG figures, both correct, differing by about a fifth. Anyone comparing cars across the two markets without converting the gallon first is comparing the sizes of two measuring jugs, not the thirst of two cars.
Worked Examples: Two Cars Converted Every Way
Example 1 — the frugal hatchback at 5 L/100 km. In kilometres per litre: 100 / 5 = 20 km/L. In MPG (US): 235.215 / 5 = 47.0 MPG. In MPG (UK): 282.481 / 5 = 56.5 MPG. One car, four figures, and note the audiences: the 56.5 a British reader sees and the 47.0 an American reader sees describe identical thirst. A seller quoting the UK figure into an American listing without a label would be flattering the car by nearly ten miles per gallon.
Example 2 — the larger family car at 8 L/100 km. In kilometres per litre: 100 / 8 = 12.5 km/L. In MPG (US): 235.215 / 8 = 29.4 MPG. In MPG (UK): 282.481 / 8 = 35.3 MPG. Now compare the pair honestly. The second car uses 8 litres where the first uses 5 over the same 100 km — sixty percent more fuel. Its MPG figures are lower in both gallons, exactly as the direction check requires: higher consumption, lower economy, every time.
Example 3 — converting back from an American figure. A US review quotes 29.4 MPG and a European reader wants litres per 100 km. Reverse the US formula: 235.215 / 29.4 = about 8.0 L/100 km, landing on Example 2's car. The round trip through the constant works in both directions because the formula is a pure reciprocal: divide the constant by whichever figure you have to get the other.
Fuel economy in MPG (US) — higher is better, lower L/100 km is better — hover or tap each bar for details.
The chart shows MPG (US) rising as L/100 km falls — the two scales running in opposite directions, drawn on one axis. Read it from the left and the trap becomes visible: the tallest bar belongs to the smallest consumption figure. Anyone who skimmed a table of mixed units and assumed taller, bigger or higher always meant thirstier would walk away with the ranking exactly backwards. Pin this picture next to any cross-market comparison and the reciprocal stops being an algebra detail and becomes something you can see.
The Reciprocal Trap, Stated Plainly
| Comparison | What a quick glance suggests | What is actually true |
|---|---|---|
| 5 L/100 km vs 47.0 MPG | The MPG car uses far more — 47 is bigger than 5 | They are the same car in two units |
| 8 L/100 km vs 5 L/100 km | The 8 car is a little thirstier | It uses 60 percent more fuel over the same distance |
| 56.5 MPG (UK) vs 47.0 MPG (US) | The UK car is much more economical | They can be the same car; the UK gallon is larger |
| 20 km/L vs 12.5 km/L | The 20 car goes further per litre | Correct — km/L runs in the same direction as MPG |
| A rise from 10 to 5 L/100 km | Consumption halved | Correct — and MPG exactly doubles, because the units are reciprocals |
The last row hides the subtlest point in this topic. Equal steps in L/100 km are not equal steps in MPG. Improving from 10 to 8 L/100 km moves US economy from 23.5 to 29.4 MPG, a gain of about 5.9. Improving from 6 to 4 L/100 km — the same two-litre saving — moves it from 39.2 to about 58.8 MPG, a gain of nearly 19.6. Consumption steps are even; economy steps stretch as cars get thriftier. That is why regulators who want honest comparisons prefer L/100 km, and why a marketer who wants a dramatic headline reaches for the MPG difference at the frugal end of the range. Converting both ways, with the Fuel Economy Converter, is how a reader keeps both descriptions honest at once.
Fuel Figures Meet Distance, Volume and the Fuel Bill
A fuel economy figure only becomes money when it meets a distance and a fuel price. The working runs through the neighbouring units in this series: plan the trip in kilometres with the Length Converter and the method in our length conversion guide, then divide the distance by 100 and multiply by the L/100 km figure to get litres needed — a 450 km trip in the 8 L/100 km car needs 4.5 x 8 = 36 litres. Tank sizes and fuel bought in other countries pull in the Volume Converter, because a gallon on a foreign fuel receipt is a volume before it is an economy figure, and our volume conversion guide separates the US and UK gallons there too. Keeping the three conversions in order — distance, consumption, volume — turns a foreign brochure figure into a realistic fuel bill for your own roads, in your own currency, without a single guessed number.
Five Mistakes That Spoil Fuel Economy Conversions
- Comparing numbers across the reciprocal divide. The founding error of this topic. A 5 beside a 47.0 is not a small car beside a big one; until both figures share a unit and a direction, no comparison exists at all.
- Mixing US and UK gallons. An unlabelled MPG figure is an unfinished fact. The two gallons differ by about a fifth, so the mix-up flatters or insults a car by roughly twenty percent — larger than the genuine difference between most rival models.
- Multiplying where the formula divides. Habits from every other converter say multiply by the factor. Here the factor is divided into or out of the constant 235.215 or 282.481. A multiplied result is usually absurd — hundreds of MPG — but smaller slips can pass unnoticed.
- Averaging MPG figures directly. Because MPG is a reciprocal scale, the average of two cars' MPG is not the economy of the pair driven together. Convert to L/100 km, average the consumption, then convert back if an MPG answer is required.
- Trusting a single figure as a promise. Published economy figures are measured under set test conditions and are best used for comparing cars against each other, not for predicting your exact fuel bill. Your roads, loads and driving will move the real figure; the conversion between units, by contrast, is exact.
Which Figure Should You Quote?
Quote the unit your audience already thinks in, and give one conversion beside it when the audience is mixed. A listing aimed at American buyers leads with MPG (US); the same car sold to British buyers leads with MPG (UK); a technical comparison across several markets is clearest in L/100 km, because its steps are even and it never flatters anyone. Whatever you lead with, print the unit in full — MPG (US), not just MPG — and resist the temptation to pick whichever converted figure happens to be largest. Readers who catch a seller quietly swapping gallons stop trusting everything else on the page, and they are right to: in a topic built on a reciprocal and two different gallons, the label on the number is where honesty lives.
Car listings, rental pages and import adverts lose credibility the moment a fuel figure appears without its unit — or with the wrong gallon quietly chosen. If your website sells or hires vehicles across markets, book a free consultation with Asad Webs and mention the listings involved; consistent, fully labelled fuel figures are a small fix that serious buyers notice immediately.
Frequently Asked Questions
How do I convert litres per 100 km to MPG?
Divide the constant for the gallon you mean by the L/100 km figure: MPG (US) = 235.215 / (L/100 km), and MPG (UK) = 282.481 / (L/100 km). So 8 L/100 km is 235.215 / 8 = 29.4 MPG (US) and 282.481 / 8 = 35.3 MPG (UK).
Why are US and UK MPG figures different for the same car?
Because the gallons differ. A UK gallon is larger than a US gallon, so the same car travels more miles on a UK gallon and earns a higher MPG figure in Britain — about 20 percent higher — without using a drop less fuel.
Is 8 L/100 km good fuel economy?
It converts to 12.5 km/L, 29.4 MPG (US) or 35.3 MPG (UK). Whether that counts as good depends on the size and type of vehicle, but the conversion lets you compare it fairly against any figure quoted in the other units, which is the point of converting before judging.
How do I convert MPG back to litres per 100 km?
Use the same constants in reverse: L/100 km = 235.215 / MPG (US), or 282.481 / MPG (UK). An American figure of 47.0 MPG gives 235.215 / 47.0 = about 5.0 L/100 km, returning you to the starting figure.
What does kilometres per litre mean, and how is it calculated?
It is the distance one litre carries the car, the mirror image of litres per 100 km. The formula is km/L = 100 / (L/100 km), so 5 L/100 km gives 100 / 5 = 20 km/L. Higher km/L is better, like MPG, and lower L/100 km is better.
Why does fuel economy use a reciprocal in some countries?
Litres per 100 km measures consumption — fuel used for a fixed distance — while MPG and km/L measure economy — distance achieved for a fixed amount of fuel. Both describe the same behaviour from opposite ends, which is why converting between them divides instead of multiplying.
Which unit should I use when comparing two cars?
Convert both cars into one unit — the one your own fuel prices and driving are recorded in — and compare like with like. Never compare one car's L/100 km figure directly against another car's MPG figure; the raw numbers run in opposite directions.
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