Fuel Economy Converter
Convert fuel economy between MPG (US), MPG (UK), L/100km, and km/L instantly.
- Free, no account
- No watermark
- No usage limit
About the Fuel Economy Converter
This converter turns any fuel economy figure into the other three units, MPG (US), MPG (UK), L/100km, and km/L, the moment you type it. Where most converters stop at the number, this one also tells you which numbers are the good ones, and for fuel economy that's genuinely the confusing part, because the units disagree on direction. Three of them, MPG (US), MPG (UK), and km/L, climb as a car gets better. The fourth, L/100km, drops. So on its own the figure 8 could be a thirsty car or a frugal one, you can't tell until you know the unit. Every value the tool shows carries a higher = better or lower = better tag, so you never rank a 5 L/100km car below an 8 by mistake. It runs entirely in your browser, updates as you type, and keeps working offline once the page has loaded, so nothing you enter ever leaves your device.
How to use
- Type your figure in the From box. Say 30, straight off a US window sticker.
- Pick what that number is in the first dropdown, MPG (US), MPG (UK), L/100km, or km/L.
- Pick the unit you want in the second dropdown. The result changes as you type, there's no convert button to press.
- Read the panel below the result. It shows the same car's economy in all four units at once, each line labeled with whether up or down is the good direction.
- Hit the swap arrow to flip From and To without retyping the value.
As a rough anchor, 30 MPG (US) is about 7.84 L/100km and 12.75 km/L, and all three describe one ordinary modern car. If a conversion lands nowhere near that scale, you've probably got the wrong From unit selected.
Why a bigger number isn't always better
Fuel economy trips people up in a way temperature never does, and the reason is that two of these units measure the opposite thing. MPG and km/L answer how far you go on a fixed amount of fuel, so more distance is the better car and the number climbs. L/100km answers how much fuel you burn over a fixed distance, so less fuel is the win and the number falls, which means one car scores opposite ways depending on which unit you read it in.
That opposition is also why you can't convert by multiplying. Miles to kilometers is a fixed multiply because both count the same way. MPG to L/100km isn't, because as one rises the other has to fall. The link is a division: L/100km = 235.215 ÷ MPG (US), and the very same 235.215 ÷ L/100km hands the MPG back. When one constant works in both directions like that, you're looking at an inverse pair, not a scale factor.
Where this actually bites is a spec sheet. You see "combined 6.2" on a European brochure and "34" on a US one, your brain wants to rank them, and they aren't even pointing the same way. That's the moment the panel is for. Drop either figure in and read all four lines with their labels, and the comparison stops being a memory test.
The US and UK gallon aren't the same size
One mistake trips up nearly every cross-border comparison, and it has nothing to do with the car. A US gallon is 3.785 liters. A UK (imperial) gallon is 4.546 liters, about a fifth bigger. Since MPG is just miles per gallon, a bigger gallon means more miles counted against each gallon, so the identical car posts a higher MPG in Britain than in America without being one bit more efficient.
And the gap is large. One MPG (UK) is roughly 0.83 MPG (US), which means a British car advertised at 40 MPG is really doing about 33 in US terms. That's enough to flip which of two cars looks cheaper to run. Anyone importing a vehicle, or reading a UK review while shopping a US-spec model, gets caught by this constantly.
Plenty of converters offer a single "MPG" and quietly assume US gallons. We keep MPG (US) and MPG (UK) as two separate units on purpose, because collapsing them into one is exactly the mistake. If you're not sure which a number is, the source usually tells you, imperial for anything British, US gallons almost everywhere an American wrote it.
Frequently asked questions
Does improving MPG by the same amount always save the same fuel?
No, and this one fools almost everyone. Because MPG counts distance per unit of fuel, a fixed jump saves wildly different amounts depending on where you start. Take a car from 10 to 20 MPG and you halve its fuel use per 100 miles, from 10 gallons down to 5. Take one from 30 to 40, the same ten-point bump, and you only drop from 3.33 gallons to 2.5. Fixing your thirstiest vehicle beats squeezing an already-frugal one, L/100km shows this plainly because it's linear, MPG buries it.
How do I convert MPG to L/100km without the tool?
Divide 235.215 by a US MPG figure, or 282.481 by a UK one. So 45 MPG (US) is 235.215 ÷ 45, about 5.2 L/100km. The same constant runs backward: 235.215 divided by an L/100km value returns US MPG. Going to or from km/L is easier still, it's 100 ÷ L/100km, no odd constant needed.
What is km/L and where is it used?
Kilometers per liter is the metric take on MPG, distance per fuel, higher is better. You'll find it on spec sheets across India, Japan, and much of Latin America and the Middle East. It sits oddly between the two camps, metric units like L/100km but the distance-per-fuel logic of MPG. A quick anchor: 20 km/L is the same economy as 5 L/100km.
Are these conversions exact?
The constants come from exact legal definitions, a US gallon is 3.785411784 liters, a UK gallon 4.54609, a mile 1.609344 km, so the arithmetic is precise well past any digit that matters for a car. Your real economy swings far more with weather, load, traffic, and how you drive than with any rounding here. Treat the converted figure as solid and your original number as the loose part.
Is a 5 L/100km car really better than an 8?
Yes, clearly. L/100km is fuel burned per 100 km, so a lower number means less fuel to cover the same ground. 5 is a genuinely efficient car, 8 is roughly average, and the higher figure is the thirstier one. This is the exact reversal the tool's labels catch for you, since with MPG and km/L the ranking runs the other way and higher wins.
Which unit should I actually use?
Whatever the people you're comparing against use. If you're cross-shopping US cars, stay in MPG (US). Budgeting fuel for a trip in Europe or Canada, L/100km is the one that turns straight into liters and money. The point of showing all four side by side is that you don't have to commit, you can read the same car in your unit and the seller's at the same time.
Does it work offline, and do my numbers stay private?
Both yes. The whole thing runs as code inside your own browser, so there's no server doing the math and nothing to upload. Once the page has loaded you can drop your connection entirely and it still converts. Whatever figures you type stay on your device, which matters more for something like a business mileage log than a one-off curiosity.