Free Fuel Economy Converter Online

A fuel economy converter lets you move a vehicle's efficiency rating between miles per gallon (US), miles per gallon (UK/imperial), liters per 100 kilometers, and kilometers per liter — the four ways fuel economy gets quoted depending on which country's spec sheet, road test, or car listing you're reading. The part that trips people up is that mpg and L/100km don't move together in a straight line: a bigger mpg number means better efficiency, while a bigger L/100km number means worse efficiency, so the two scales run in opposite directions and the conversion between them is a division, not a simple multiplication. This fuel economy converter handles that reciprocal relationship — and the fact that the US and UK gallon aren't even the same size — automatically, so you get the right number without having to remember which way the math runs.

How the Fuel Economy Converter Works

On the tool page, you enter a single value in liters per 100 kilometers and the result comes back as all three of the other units at once, so you can see US mpg, UK mpg, and km/L side by side from one entry.

• Enter your L/100km figure — this is the tool's reference unit for the on-page display.
• The result box returns the equivalent in miles per gallon (US), miles per gallon (UK), and kilometers per liter simultaneously, so there's no need to run three separate conversions.
• If your starting number is in mpg rather than L/100km, the API version of this converter (linked below) accepts any of the four units as the starting point and any other as the target, rather than requiring L/100km as the entry point.

All of the arithmetic happens on your own device — nothing about the number you type is sent off anywhere to be processed. For a workflow where you're pulling fuel-economy figures from a database or a form submission and need the conversion done server-side instead of by hand, this converter is also available as an API endpoint, with full parameter documentation at /docs.

Why You'd Convert Fuel Economy Figures

Fuel economy conversion mostly comes up at the exact moment two different labeling conventions collide:

• Comparing an imported or overseas car listing. A European or Japanese vehicle spec sheet will quote L/100km, while most US buyers think in mpg — converting lets you judge whether "5.5 L/100km" is actually a good or mediocre number without doing mental division.
• Planning a road trip abroad. If you're renting a car in the UK or continental Europe and budgeting fuel costs, your rental's spec sheet and the fuel pump price will both be in metric or L/100km terms, while your mental reference point is probably mpg from home.
• Reading UK-market reviews or spec sheets. The UK and Ireland use "mpg" too, but it's the imperial gallon, which is meaningfully larger than the US gallon — treating a UK mpg figure as if it were a US mpg figure overstates how efficient the car actually is.
• Evaluating hybrid or EV-adjacent efficiency claims. Manufacturers marketing globally often publish the same vehicle's efficiency in whichever unit is standard for each market, and lining up the figures side by side is the only way to tell if the underlying performance is actually consistent.
• Fleet or logistics planning across regions. A company running vehicles in both the US and a metric-using country needs a common basis for comparing fuel costs and efficiency across the fleet, which means converting everything into one shared unit first.
• Cross-checking an online marketplace or classifieds listing. Used-car listings sourced from a different country sometimes get re-posted with the original fuel economy figure left untouched, and a quick check confirms whether that "4.5" you're looking at is L/100km or something else entirely before you factor it into an offer.

Technical Deep Dive: Why mpg and L/100km Aren't a Straight Conversion

Miles per gallon measures distance per unit of fuel — more is better. Liters per 100 kilometers measures fuel per unit of distance — less is better. Because one is a "distance over fuel" ratio and the other is a "fuel over distance" ratio, converting between them means flipping the fraction, and the mathematical result is that L/100km equals a fixed constant divided by mpg, not a constant multiplied by mpg. That single fact is the whole reason a fuel economy converter is more useful than a simple multiply-by-a-factor approach: applying a flat percentage or linear factor between the two scales will quietly produce a wrong number.

The practical consequence is that equal steps in mpg do not translate into equal steps in L/100km. Using the exact conversion constant (100 km's worth of US gallons divided by the km-per-mile figure, which works out to about 235.21), a car improving from 20 mpg to 40 mpg drops from roughly 11.76 L/100km to roughly 5.88 L/100km — a full 50% cut in fuel burned per 100 km. But the next equal step, from 40 mpg to 60 mpg, only brings L/100km down from about 5.88 to about 3.92 — a much smaller additional saving for the same 20-mpg gain. Going from a low mpg to a slightly higher one saves far more real fuel than the identical mpg improvement does further up the scale, which is exactly what you'd expect once you see that L/100km is a reciprocal of mpg rather than a linear twin of it.

The US and UK/imperial gallon add a second wrinkle on top of the reciprocal relationship: the US gallon is about 3.785 liters, while the UK imperial gallon is about 4.546 liters — roughly 20% larger. That means a car with identical real-world fuel efficiency will show a noticeably higher mpg number when quoted in UK gallons than in US gallons, purely because the UK gallon is a bigger unit of fuel, and each one carries the car further. A vehicle rated at 30 mpg (US) converts to roughly 36 mpg (UK) for the exact same underlying efficiency — not because the car performs any differently, but because the two "gallons" being divided into aren't the same size.

Unit	Direction of "better"	Notes
mpg (US)	Higher is better	Based on the 3.785-liter US gallon
mpg (UK)	Higher is better	Based on the larger 4.546-liter imperial gallon
L/100km	Lower is better	Fuel volume needed to cover a fixed 100 km distance
km/L	Higher is better	Distance covered per liter — a direct, non-reciprocal metric-to-metric read

It's worth noting that km/L and L/100km are also reciprocals of each other, just both expressed in metric terms rather than one metric and one imperial: km/L equals 100 divided by L/100km, and vice versa. So a car rated at 100 km/L would use exactly 1 L/100km, and one rated at 20 km/L would use 5 L/100km — the same flip-the-fraction logic applies even when both sides of the conversion are metric, it's simply less noticeable than the mpg case because km/L and L/100km tend to move in more familiar-looking ranges for most everyday cars.

One more caveat worth stating plainly: a fuel-economy rating printed on a spec sheet is typically a combined, city, or highway figure produced under a standardized test cycle, and real-world mileage on any given tank of fuel will vary with driving style, terrain, temperature, and load. Converting a rating between units doesn't change how accurately that original rating reflects your actual driving — it only re-expresses the same underlying number in a different unit so you can compare it against a figure quoted a different way.

Fuel Economy Converter vs Other Approaches

• Rule-of-thumb mental math. A common shortcut is "divide 235 by the mpg figure," which does work for US mpg to L/100km, but it's easy to misapply the same shortcut to UK mpg without adjusting for the larger gallon, or to apply a linear percentage change instead of correctly re-dividing.
• A spreadsheet with a hardcoded formula. Workable once it's set up correctly, but a common mistake is building the formula as a straight multiplication (treating the units as linearly related) rather than a division — that error is easy to introduce and easy to miss until the numbers look obviously wrong.
• Manufacturer or government fuel-economy websites. Useful for looking up a specific vehicle's rating in its home market's unit, but they don't necessarily convert that figure into the unit you actually need for comparison.
• A dedicated fuel economy converter. Applies the correct reciprocal math and the correct US/UK gallon distinction automatically every time, which matters most exactly when you're least likely to catch a manual mistake — mid-comparison, juggling more than one vehicle's numbers at once.

Common Questions About Fuel Economy Conversion

Why isn't converting mpg to L/100km just a multiplication?

Because the two units measure fuel economy from opposite directions — one is distance per fuel, the other is fuel per distance — the conversion is a division (a fixed constant divided by the mpg value), not a linear scale factor.

Is a UK mpg figure the same as a US mpg figure?

No. They use the same phrase but different-sized gallons — the UK imperial gallon is about 20% larger than the US gallon — so a UK mpg number will read noticeably higher than a US mpg number for the identical real-world fuel efficiency.

Is a lower L/100km number always better, or is higher better?

Lower is better for L/100km, since it measures how much fuel is consumed to cover a fixed 100 km distance — the less fuel needed, the more efficient the vehicle.

Do hybrid and electric vehicles get quoted in these same units?

Hybrids with a gasoline engine are typically rated in mpg or L/100km just like conventional cars. Fully electric vehicles use different efficiency units entirely (like miles per kWh or kWh per 100 km), which this fuel economy converter does not cover, since it's built specifically around gallon- and liter-based fuel economy.

Why does a 20-mpg improvement matter more at low mpg than at high mpg?

Because L/100km is the reciprocal of mpg, equal mpg increases produce shrinking L/100km reductions as the starting mpg gets higher — going from 20 to 40 mpg saves far more fuel per 100 km than going from 40 to 60 mpg, even though both represent the same 20-mpg jump.

Does a fuel economy conversion account for combined, city, or highway ratings?

No — the conversion applies to whatever single figure you enter, regardless of whether that figure originally came from a combined, city, or highway test cycle. Converting the unit doesn't change which testing condition the underlying number represents, so it's worth keeping track of which rating type you're comparing on each side.

Why do two cars with the same mpg sometimes show different L/100km figures online?

That's usually a sign one listing is quoting US mpg and the other UK mpg without saying so — since the UK gallon is roughly 20% larger, the same L/100km value converts to two different-looking mpg numbers depending on which gallon definition is being used, which is exactly the mix-up this fuel economy converter is built to catch.

Related Tools

If you're working out fuel costs or trip planning alongside a fuel-economy figure, the Length Converter is useful for converting trip distances between miles and kilometers, and the Volume Converter handles gallons, liters, and other fuel-tank capacity units for budgeting a fill-up. The Speed Converter comes in handy when a vehicle's efficiency rating needs to be considered alongside speed limits quoted in a different unit for an overseas trip, and the Weight Converter is worth a look if payload or towing weight (often quoted in a different unit than your home country uses) factors into the comparison. Anyone integrating fuel-economy conversion into a fleet dashboard or booking tool rather than checking figures by hand can find the full API reference at /docs.