L/100km, km/L, mpg, kWh/100km, MPGe — what each unit means, why they trip people up, and how to read them side by side.
Most efficiency units work the intuitive way — a higher number means a more efficient car. A car doing 20 km/L beats one doing 10 km/L; 50 mpg beats 25 mpg. L/100km is the opposite: it measures consumption, not output. It tells you how many litres the engine burns to travel 100 km. Less fuel burned = smaller number = better car.
The two are always related by a simple reciprocal:
There is a second trap: improving a thirsty car saves more fuel than the same absolute improvement on an already-efficient one. Going from 10 L/100km to 8 L/100km saves 2 litres per 100 km. Going from 6 to 4 also saves 2 litres — the same number, but a much larger percentage reduction. This non-linearity is invisible in mpg or km/L terms but becomes obvious once you think in L/100km.
| L/100km | km/L | mpg (US) | mpg (UK) |
|---|---|---|---|
| 4 | 25.0 | 58.8 | 70.6 |
| 5 | 20.0 | 47.0 | 56.5 |
| 6 | 16.7 | 39.2 | 47.1 |
| 7 | 14.3 | 33.6 | 40.4 |
| 8 | 12.5 | 29.4 | 35.3 |
| 10 | 10.0 | 23.5 | 28.2 |
| 12 | 8.3 | 19.6 | 23.5 |
| 15 | 6.7 | 15.7 | 18.8 |
Highlighted row: typical efficient compact car. Formulas: mpg (US) = 235.21 ÷ L/100km; mpg (UK) = 282.48 ÷ L/100km.
The US and UK both express fuel economy in "miles per gallon" but use different gallon sizes. The divergence is historical:
Because the UK gallon holds about 20% more, UK mpg figures are always about 20% higher for the same car:
| mpg (US) | mpg (UK) | L/100km |
|---|---|---|
| 20 | 24.0 | 11.76 |
| 25 | 30.0 | 9.41 |
| 30 | 36.0 | 7.84 |
| 33 | 39.6 | 7.13 |
| 40 | 48.0 | 5.88 |
| 50 | 60.0 | 4.70 |
| 60 | 72.1 | 3.92 |
Always check which standard a manufacturer or publication uses. European and Australian specs quote L/100km; US official figures use US gallons; UK official figures use imperial gallons.
MPGe — Miles Per Gallon equivalent — was introduced by the US EPA so that EV buyers could compare their car to a petrol car using a number they already understood. The bridge is an energy equivalence:
33.7 kWh is the EPA's rounded figure for the lower heating value (LHV) of one US gallon of regular unleaded gasoline — the actual usable energy released when it burns. Given that, the formula is:
Electric motors convert roughly 85–90% of stored energy into motion; a petrol engine converts only 20–35%. This is why even an average EV scores well above 100 MPGe — the unit captures the fundamental efficiency advantage of electric drive, not just range.
| kWh/100km | km/kWh | mi/kWh | MPGe |
|---|---|---|---|
| 13 | 7.7 | 4.8 | 161 |
| 15 | 6.7 | 4.1 | 140 |
| 17 | 5.9 | 3.7 | 123 |
| 20 | 5.0 | 3.1 | 105 |
| 23 | 4.3 | 2.7 | 91 |
Highlighted row: typical mainstream compact EV. Most petrol cars score 20–50 MPGe on the same scale.
Manufacturer test-cycle figures (WLTP in Europe, EPA in the US) typically run 10–20% better than on-road averages. The ranges below reflect real-world observed consumption.
| Vehicle type | L/100km | mpg (US) | mpg (UK) |
|---|---|---|---|
| Small city car | 5.0 | 47.0 | 56.5 |
| Efficient compact | 5.5 | 42.8 | 51.4 |
| Average family saloon | 7.0 | 33.6 | 40.4 |
| Diesel SUV | 7.5 | 31.4 | 37.7 |
| Large petrol SUV | 11.0 | 21.4 | 25.7 |
| Performance / sports car | 13.0 | 18.1 | 21.7 |
| Vehicle type | kWh/100km | km/kWh | mi/kWh | MPGe |
|---|---|---|---|---|
| Efficient compact EV | 13 | 7.7 | 4.8 | 161 |
| Mainstream compact EV | 15 | 6.7 | 4.1 | 140 |
| Mid-size EV saloon | 17 | 5.9 | 3.7 | 123 |
| Large EV SUV | 20 | 5.0 | 3.1 | 105 |
| Performance EV (AWD) | 23 | 4.3 | 2.7 | 91 |
Instant two-way converters for every fuel economy pair:
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L/100km measures fuel consumption — how many litres the engine burns to travel 100 km. Because it measures waste rather than output, a lower number means a more efficient car. A car using 5 L/100km is twice as efficient as one using 10 L/100km. The formula linking it to km/L is: km/L = 100 ÷ L/100km.
Because the UK uses a larger gallon. One US gallon = 3.785 L; one UK (imperial) gallon = 4.546 L — about 20% more. The same car always shows a higher mpg figure in the UK. Convert with: mpg (UK) = mpg (US) × 1.2009. A car rated 40 mpg in the US shows approximately 48 mpg in the UK.
MPGe (Miles Per Gallon equivalent) is a US EPA unit for comparing EVs to petrol cars on a single scale. The EPA equates 33.7 kWh to the energy in one US gallon of petrol. MPGe = (mi/kWh) × 33.7 or, starting from the European unit, MPGe ≈ 2094 ÷ kWh/100km. An EV using 15 kWh/100km achieves about 140 MPGe.
A small efficient petrol car typically achieves 5–6 L/100km (39–47 mpg US / 47–57 mpg UK). An average family saloon uses 7–8 L/100km (29–39 mpg US). A large petrol SUV uses 10–13 L/100km (18–24 mpg US). A mainstream compact EV typically uses 14–17 kWh/100km (120–140 MPGe), and a large EV SUV uses 18–22 kWh/100km.