Online Tool Store
· 5 min read
How to Convert MPG to L/100km (and Back)
Heshan Fernando
Co-founder & COO
You’re comparing a US car review that quotes “32 MPG combined” against a European listing that quotes “7.3 L/100km,” and you genuinely can’t tell which car is more efficient without doing math you don’t remember from school. It gets worse when you throw in a UK MPG figure, because UK gallons and US gallons aren’t the same size — a “40 MPG” UK car and a “40 MPG” US car use noticeably different amounts of fuel.
This isn’t a rare edge case. Anyone shopping for a car online, renting a vehicle abroad, or reading an import spec sheet runs into mixed fuel economy units almost immediately, and the units don’t just use different scales — some of them are inverted, so “higher is better” flips to “lower is better” depending which one you’re looking at.
What fuel economy units actually measure
MPG (miles per gallon) tells you distance per unit of fuel — bigger numbers mean better efficiency. L/100km flips that logic: it tells you fuel consumed per 100 kilometers traveled, so smaller numbers mean better efficiency. Km/L is closer to MPG in spirit — distance per unit of fuel, just in metric units.
The MPG trap is the gallon itself. A US gallon is about 3.785 liters; a UK (Imperial) gallon is about 4.546 liters — roughly 20% larger. That means “35 MPG” in the US and “35 MPG” in the UK describe genuinely different fuel efficiency, not just a labeling difference.
Why people get stuck here
- US MPG and UK MPG look identical but aren’t. Nothing on the label tells you which gallon is being used unless the source explicitly says “US” or “Imperial.”
- L/100km runs backwards from MPG. Someone used to “bigger is better” instinctively misreads a lower L/100km number as worse, when it’s actually more efficient.
- Manufacturer spec sheets mix units by region. A US import listing might quote L/100km from the original market without converting, leaving the buyer to do it manually.
- Rental car comparisons rarely normalize units. Booking a car abroad often means comparing a US-spec listing against a local L/100km rating with no common unit in sight.
What a reliable mileage converter should handle
All four common fuel economy units
US MPG, UK MPG, L/100km, and km/L cover the vast majority of listings you’ll actually run into — a converter missing UK MPG specifically will fail you the moment you’re comparing a British car spec.
Correct handling of the inverse relationship
Because L/100km is consumption-per-distance while MPG and km/L are distance-per-consumption, a naive linear conversion is wrong. The tool needs to actually invert the relationship, not just rescale it.
Clear labeling of which gallon is in use
If “MPG” doesn’t specify US or Imperial, the number is ambiguous. A converter should force you to pick one rather than silently assuming.
| Metric | Direction | Better Means |
|---|---|---|
| US MPG | Distance per US gallon | Higher number |
| UK MPG | Distance per Imperial gallon | Higher number |
| km/L | Distance per liter | Higher number |
| L/100km | Liters per 100 km traveled | Lower number |
Common mistakes to avoid
- Comparing a US MPG figure to a UK MPG figure as if the gallons match — they’re about 20% apart.
- Reading a lower L/100km number as “worse” out of MPG habit, when it actually means better efficiency.
- Converting MPG to L/100km with a simple division instead of the correct inverse formula.
- Using a “combined” rating from one region against a “city” or “highway” rating from another without matching the driving condition.
- Rounding too early in multi-step conversions (MPG to km/L to L/100km), which compounds small errors into a noticeably wrong final number.
How to do it with Mileage Unit Converter
Online Tool Store’s Mileage Unit Converter runs entirely in your browser and handles all four units, including the US/UK MPG distinction.
- Open the Mileage Unit Converter and select your starting unit — US MPG, UK MPG, L/100km, or km/L.
- Enter the figure from the listing, spec sheet, or rental agreement you’re comparing.
- Read the converted values in the other three units instantly.
- Repeat for the second vehicle so both are in the same unit before you compare.
Frequently asked questions
Is UK MPG the same as US MPG?
No. A UK (Imperial) gallon is about 4.546 liters, versus about 3.785 liters for a US gallon — roughly 20% larger. The same car will show a higher MPG figure under the UK measure than under the US measure, even though nothing about the car changed.
Why does a lower L/100km number mean better efficiency?
L/100km measures fuel consumed per 100 kilometers driven, so it’s a consumption rate, not a distance rate. Using less fuel to cover the same distance is more efficient, which is why the smaller number wins — the opposite of how MPG works.
What’s a “good” L/100km figure for a regular car?
It depends heavily on vehicle size and driving conditions, so there’s no single universal number — but converting to a common unit at least lets you compare two specific cars fairly, which is usually the actual question people are trying to answer.
Final thought
Fuel economy units aren’t just relabeled versions of each other — one of them runs backwards, and two of them use gallons of different sizes. Convert everything to one unit before you compare, and don’t trust a “MPG” figure that doesn’t say which gallon it means.