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TravelA car rated 40 MPG in America and a car rated 5.9 L/100km in Europe are describing the same efficiency — you'd never know it from the numbers. The two systems are reciprocals of each other, and worse, one of them flips the meaning of "higher is better."
Here's how both systems work, the exact conversion, and the gallon-size trap that makes British MPG numbers look better than they are.
Try it yourself: Convert instantly between MPG, L/100km, and km/L. Fuel Economy Converter →
The two systems
- MPG (miles per gallon) — distance per unit of fuel. Higher is better. Used in the US (and UK, with a bigger gallon).
- L/100km (liters per 100 km) — fuel per unit of distance. Lower is better. Used in Europe, Canada, Australia, and most of the world.
The L/100km system is arguably more honest: fuel consumption scales linearly with cost. Going from 10 to 8 L/100km saves exactly as much fuel as going from 8 to 6 — while in MPG, the same improvement looks wildly different at different ends of the scale.
The conversion formula
L/100km = 235.214 ÷ MPG (US)
MPG (US) = 235.214 ÷ L/100km
MPG (US) = 235.214 ÷ L/100km
| MPG (US) | L/100km |
|---|---|
| 20 | 11.76 |
| 30 | 7.84 |
| 40 | 5.88 |
| 50 | 4.70 |
The US vs. UK gallon trap
The imperial gallon is 1.20095 US gallons — about 20% bigger. So "40 MPG" in a British review equals only 33.3 MPG in American terms. When comparing cars across the Atlantic, convert with 282.481 instead of 235.214 (that's 235.214 × 1.20095):
L/100km = 282.481 ÷ MPG (UK/imperial)
What about electric cars?
EVs use kWh/100km (or miles/kWh) — energy per distance, same logic as L/100km. A typical EV uses 15–20 kWh/100km. The US EPA also publishes "MPGe" (miles per gallon equivalent), where 33.7 kWh of electricity = 1 gallon of gasoline in energy terms — useful for comparing EVs to gas cars on one scale.
Real-world vs. rated economy
Official figures come from standardized lab cycles, and real driving typically returns 10–30% worse — short trips, cold weather, speed, and AC all cost fuel. Use ratings to compare vehicles, not to predict your fuel bill. The most accurate number for your car is your own: track miles driven between fill-ups divided by gallons added over a few tanks.
Cost per mile: the number that matters
Cost per mile = fuel price per gallon ÷ MPG
— or: (fuel price per liter × L/100km) ÷ 100 —
— or: (fuel price per liter × L/100km) ÷ 100 —
At $3.50/gal and 30 MPG, each mile costs 11.7¢. A 15,000-mile year = $1,750 in fuel. Drop to 20 MPG and it's $2,625 — the 10-MPG gap costs $875 a year. This is why L/100km-style thinking wins for budgeting: consumption maps linearly to money.
km/L: the third option
Some countries (Japan, India, parts of South America) use km/L — distance per volume, like MPG but metric. Higher is better, and conversion is easy: km/L = 100 ÷ L/100km. A car at 5 L/100km does 20 km/L. All three systems describe the same physics; pick whichever your fuel receipts use.
Hybrids and plug-in hybrids
Hybrids quote a single combined figure, but plug-in hybrids (PHEVs) have two modes: electric and gasoline. Their headline economy assumes a specific electric/gas split that may not match your driving — a PHEV driven mostly on short electric trips can effectively use near-zero fuel, while one never plugged in is just a heavy hybrid. For PHEVs, kWh/100km plus L/100km in hybrid mode tells the real story; the blended headline number rarely does.
Reading the sticker: combined vs. highway vs. city
Economy labels show three numbers for a reason: city driving (stop-and-go) is always thirstier than highway cruising. A car rated 25 city / 35 highway does 30 combined — but if your life is all short urban trips, budget on the city figure, not the combined one. Hybrids flip the pattern: regenerative braking makes them more efficient in the city than on the highway, the opposite of conventional cars.
Frequently asked questions
Lower. L/100km measures fuel consumed per distance, so less fuel for the same trip is better. (It's the opposite of MPG, where higher is better.)
Because the imperial gallon (4.546 L) is ~20% larger than the US gallon (3.785 L). A car doing 40 imperial MPG does 33.3 US MPG — same efficiency, bigger gallon.
Roughly: under 6 is excellent, 6–8 is good, 8–10 is average for a modern car, and over 12 is thirsty. Hybrids routinely beat 5; large SUVs sit around 10–14.
In kWh/100km (energy per distance) — lower is better, just like L/100km. The US also uses MPGe, where 33.7 kWh equals one gallon of gasoline in energy content.
Disclaimer: Official economy figures come from lab test cycles and typically overstate real-world efficiency by 10–30%. Use them to compare cars, not to predict your fuel bill exactly.