The fuel economy converter above turns any mileage figure — US MPG, Imperial MPG, liters per 100 kilometers, kilometers per liter, or miles per liter — into every other unit at once, so you can compare a US window sticker against a European brochure or a Canadian road-trip estimate without doing the math by hand.
We built this tool because Arb Digital works with automotive, logistics, and travel clients whose audiences constantly bump into mismatched fuel-economy units, and a clean, ad-free converter is something we wanted on our own site as well as in the content we write for clients.
What This Fuel Economy Converter Does
Type a number into the box, tell it which unit that number is already in, and it instantly recalculates the other four. It works in both directions — start from MPG and get L/100km, or start from L/100km and get MPG — because internally the fuel economy converter treats "liters per 100 kilometers" as the master unit and derives everything else from it using the distance and volume constants published by NIST and the metric standards bodies.
This matters more than it sounds. Two vehicles that both improve their MPG by exactly 5 do not save the same amount of fuel, because MPG measures how far you go per unit of fuel, while L/100km measures how much fuel you burn per unit of distance — they are reciprocals of each other, not two flavors of the same straight-line scale.
How to Use It
- Find your number. Check your car's window sticker, owner's manual, a rental agreement, or a foreign vehicle listing — it will be labeled MPG, L/100km, or km/L.
- Pick the matching unit in the dropdown. If the source is European or Canadian, it is almost always L/100km. Australian and New Zealand figures are usually also L/100km. UK-market cars sometimes still quote Imperial MPG.
- Read the four converted values in the result grid, and the big headline L/100km number at the top.
- Compare vehicles by converting them all to the same unit first — never compare a US MPG number directly to a UK MPG number, since the gallons themselves are different sizes.
The Formula — Why L/100km Is Inverse
US MPG measures miles traveled per US gallon (3.785 liters). Imperial MPG measures miles traveled per Imperial gallon (4.546 liters) — about 20% larger, which is why the same car reports a higher Imperial MPG number than US MPG for identical real-world performance. L/100km measures liters of fuel burned to cover 100 kilometers, which runs the opposite direction: as efficiency improves, L/100km goes down, not up.
The conversion routes everything through kilometers per liter as an intermediate step: km/L = (miles-per-gallon × kilometers-per-mile) ÷ liters-per-gallon, using 1 mile = 1.609344 km. Once you have km/L, L/100km is simply 100 divided by km/L, and miles per liter is km/L divided by 1.609344. Because of that division, a fixed change in MPG does not correspond to a fixed change in L/100km — the relationship curves, which is exactly the mistake this fuel economy converter is built to prevent. For the underlying unit definitions, the NIST unit conversion reference is a solid authority.
US MPG vs Imperial MPG — Why They're Not the Same Number
This is the single most common fuel-economy mix-up online. A car rated at 40 MPG in a UK review and 40 MPG in a US review is not equally efficient — the UK figure uses the larger Imperial gallon, so it represents roughly 33 MPG US. If you're reading an imported-car forum, a British motoring magazine, or an old Commonwealth-market brochure, assume Imperial MPG unless the source explicitly says otherwise, and run it through this fuel economy converter before comparing it to anything quoted in the US.
Canada is the trickiest case: pump prices and government fuel-economy labels there are quoted in L/100km, matching Europe, even though colloquial conversation sometimes still references MPG informally — and when it does, Canadians usually mean the Imperial gallon, a holdover from before metrication.
Common Fuel Efficiency Benchmarks
These illustrative figures — not guarantees for any specific model — help you sanity-check a conversion result:
| Vehicle type | Typical MPG (US) | Typical L/100km |
|---|---|---|
| Compact hybrid sedan | 50 MPG | 4.7 |
| Compact gas sedan | 35 MPG | 6.7 |
| Midsize sedan | 30 MPG | 7.8 |
| Crossover SUV | 26 MPG | 9.0 |
| Full-size SUV | 20 MPG | 11.8 |
| Pickup truck | 18 MPG | 13.1 |
| Small diesel hatchback (EU-market) | — | 4.5 |
Notice how the L/100km column doesn't move in even steps even though the MPG column does — another reminder that these two scales bend in opposite directions.
Why Fuel Economy Comparisons Trip People Up
Beyond the US/Imperial gallon gap, fuel economy figures also depend on testing cycle. US EPA combined ratings, the European WLTP cycle, and older NEDC figures are all measured differently, so even after you've converted units correctly, a European L/100km label and a US EPA-derived L/100km figure for a similar car can still diverge by 10–15% because the driving cycles behind them are not identical. This fuel economy converter fixes the unit-math problem; it can't fix differences in how each region tests vehicles, so treat cross-region comparisons as directional, not exact.
Another frequent error is averaging MPG across a trip the wrong way. If you drive 100 miles at 20 MPG and another 100 miles at 40 MPG, your combined average is not 30 MPG — it's closer to 26.7 MPG, because you used twice as much fuel on the slower leg. Converting both legs to gallons used (or to L/100km, which is already a "fuel per distance" unit) before averaging avoids this trap entirely.
Arb Digital builds fast, SEO-ready websites and free interactive tools that bring in organic traffic — this converter is one example of what we ship for clients.
Talk to Arb Digital All Free ToolsCommon Mistakes to Avoid
- Comparing a US MPG figure directly against a UK/Imperial MPG figure without converting first.
- Assuming L/100km scales linearly with MPG — it's an inverse relationship, so equal MPG gaps are not equal L/100km gaps.
- Averaging MPG across multiple trip legs by simple arithmetic mean instead of weighting by fuel used.
- Treating a EU WLTP-cycle number and a US EPA number as directly comparable once converted — the test cycles differ, not just the units.
- Forgetting that km/L and mi/L both measure distance-per-fuel (like MPG), while L/100km measures fuel-per-distance (the opposite direction).
Related Free Tools From Arb Digital
If you found this useful, you may also want the length converter for the miles-to-kilometers math behind these figures, the volume converter for gallons and liters generally, our cooking measurement converter for kitchen-specific volume conversions, and the shoe size converter and clothing size converter for other everyday US-vs-international conversions. Browse everything in our free online tools hub.
The MPG Illusion: Why Small Numbers Matter More Than Big Ones
Because MPG is a distance-per-fuel ratio rather than a straight scale, upgrading from a low MPG figure saves far more fuel than the same-sized upgrade at a high MPG figure — even though both look identical as a jump in the number. Trade a 15 MPG truck for a 20 MPG truck over 10,000 miles and you cut fuel use from about 667 gallons to 500, a saving of roughly 167 gallons. Trade a 30 MPG sedan for a 40 MPG sedan over the same distance and you go from about 333 gallons to 250, a saving of only 83 gallons — half as much fuel saved, despite the MPG jump being larger in absolute terms (10 MPG versus 5 MPG). This is often called the "MPG illusion," and it's a direct consequence of the same inverse relationship this fuel economy converter is built around: because MPG bends the fuel-per-distance curve, equal-looking MPG gains at the low end of the scale represent dramatically more real fuel saved than equal-looking gains near the high end.
The practical takeaway is that fleet buyers, city planners, and anyone comparing upgrade options should think in L/100km or gallons-per-mile terms rather than MPG whenever they're deciding where an efficiency improvement will actually matter most. A rideshare or delivery fleet swapping its least-efficient vehicles for merely "decent" ones will usually save more total fuel than the same fleet swapping its already-efficient vehicles for the most efficient ones on the market — which is why some regulators and fuel-economy labels have shifted toward gallons-per-100-miles or L/100km style figures specifically to correct for this illusion. Running any pair of before-and-after MPG numbers through this fuel economy converter and comparing the L/100km values side by side makes the real-world saving obvious in a way the raw MPG numbers alone tend to hide.
Why Rated Fuel Economy Rarely Matches Your Actual Mileage
Even after converting units correctly, drivers are often surprised that their real-world fuel economy falls short of the rated figure on the window sticker — and the gap usually has nothing to do with unit confusion. Highway speed is one of the biggest factors: aerodynamic drag increases with the square of speed, so cruising at 80 mph instead of 65 mph can burn noticeably more fuel per mile even though the engine and gearing haven't changed. Short trips and cold starts are another major factor, since an engine and catalytic converter operating below normal temperature run less efficiently, and stop-and-go city driving with a cold engine can post considerably worse figures than the same car achieves once fully warmed up on a longer highway run.
Tire pressure, cargo weight, and accessory use round out the list. Underinflated tires increase rolling resistance and quietly cost real fuel economy over time, roof racks and heavy cargo add both weight and drag, and running the air conditioning or driving with windows down at highway speed both impose a measurable efficiency penalty in different ways (AC draws engine power directly; open windows add drag). Official rated figures — whether US EPA, European WLTP, or otherwise — are produced under controlled laboratory conditions specifically designed to be repeatable and comparable between vehicles, not to replicate any individual driver's actual mix of short trips, cold mornings, hills, and highway speeds. Treat this fuel economy converter's output, and any rated MPG or L/100km figure it's converting, as a standardized comparison baseline rather than a promise of what you'll see at the pump.
Frequently Asked Questions
Imperial MPG is always the higher number for an identical vehicle, because the Imperial gallon (4.546 liters) holds about 20% more fuel than the US gallon (3.785 liters), so the same distance is covered per "gallon" using less of a US gallon's worth of fuel per mile — meaning more miles per the larger Imperial gallon.
Because L/100km measures how much fuel you burn to cover a fixed distance of 100 kilometers — using less fuel to cover that same distance means better efficiency, so the number goes down as economy improves, the opposite direction from MPG.
No. Because the two units are reciprocals of each other, the correct formula divides a constant by the MPG value rather than multiplying it, which is why this fuel economy converter routes every conversion through km/L as an intermediate step instead of using a single flat multiplier.
Compact hybrids commonly land near 50 MPG US (about 4.7 L/100km), while comparable gas-only compact sedans typically land near 35 MPG US (about 6.7 L/100km) — these are illustrative benchmarks, not guarantees for any specific model or driving style.
No — it converts units accurately (MPG, L/100km, km/L, mi/L) but cannot correct for differences between testing cycles such as the US EPA combined cycle and the European WLTP cycle, which can independently shift the underlying rating by 10–15% for a similar vehicle.
Because MPG is a distance-per-fuel ratio, simple averaging over-weights the leg where you used less fuel; the mathematically correct combined average weights each leg by fuel actually consumed, which is closer to what you get by converting to L/100km before averaging.
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