Fuel Economy: MPG vs L/100km Explained

Two very different ways to express fuel economy

The United States and the UK report fuel economy as miles per gallon (mpg) — how far a car travels on a fixed amount of fuel, where a bigger number means better efficiency. Most of the rest of the world, including the EU, reports liters per 100 kilometers (L/100km) — how much fuel a car consumes over a fixed distance, where a smaller number means better efficiency. The two metrics are not just different units, they are inverted in meaning: one counts distance per fuel, the other counts fuel per distance.

This inversion is exactly why you cannot do a simple unit conversion between mpg and L/100km the way you would between, say, miles and kilometers — you have to flip the ratio (take the reciprocal) as part of the conversion, not just multiply by a constant factor.

The conversion formula, with worked examples

The formula to convert US mpg to L/100km is: L/100km = 235.215 ÷ mpg. Worked example: a car rated at 35 mpg in the US consumes 235.215 ÷ 35 ≈ 6.72 L/100km. To go the other direction: mpg = 235.215 ÷ (L/100km). A European car rated at 5.5 L/100km achieves 235.215 ÷ 5.5 ≈ 42.8 mpg.

The constant 235.215 comes from combining the US gallon (3.78541 liters) with the conversion between miles and kilometers (1 mile = 1.60934 km), reduced to a single number so you do not need to work through both conversions separately every time. Note this formula assumes the US gallon — using it with a UK imperial-gallon mpg figure will give a slightly wrong answer, off by about 20%, unless you first convert the imperial mpg figure to US mpg.

US mpg vs UK mpg — a hidden trap

Even when both figures say “mpg,” a UK mpg figure and a US mpg figure for the identical car are not the same number, because the underlying gallon differs: the US gallon is 3.785 liters while the UK imperial gallon is 4.546 liters. A car that gets 40 mpg in US terms would show roughly 48 mpg if the identical fuel consumption were expressed using the larger UK imperial gallon — an apparent 20% efficiency “gain” that comes entirely from the unit definition, not the car.

This is one of the most common fuel-economy mistakes travelers and car shoppers make when comparing a US car review to a UK one, or vice versa. Always convert both figures to L/100km — a metric, gallon-independent standard — before comparing efficiency across sources from different countries.

Common fuel economy reference points

• 3–5 L/100km (47–78 mpg US) — small hybrid or efficient diesel cars
• 6–8 L/100km (29–39 mpg US) — typical compact/midsize gasoline sedan
• 9–12 L/100km (20–26 mpg US) — SUVs and larger family cars
• 13+ L/100km (under 18 mpg US) — trucks, large SUVs, high-performance vehicles
• Electric vehicles use a different metric entirely — kWh per 100 km, or miles per kWh — since there is no liquid fuel volume to measure

Why L/100km is arguably the more useful metric

L/100km scales linearly with actual fuel cost and consumption, which makes it easier to compare savings directly: a car using 4 L/100km instead of 8 L/100km burns exactly half the fuel over the same distance, a relationship that is immediately obvious from the numbers themselves. With mpg, the same real-world difference is much less intuitive — going from 8 L/100km to 4 L/100km corresponds to going from about 29 mpg to about 59 mpg, and the jump from 29 to 59 does not visually convey “half the fuel” the way 8 to 4 does.

This non-linearity is a known quirk of the mpg system: improving from 10 mpg to 20 mpg saves far more absolute fuel over a fixed distance than improving from 40 mpg to 50 mpg, even though both represent a 10 mpg increase — a fact that surprises many people used to thinking in mpg terms, and one reason some US publications have begun including gallons-per-100-miles alongside mpg for clarity.

Practical tips for comparing cars across countries

When researching a car online, note which fuel economy standard the source uses — EPA (US), WLTP (the current EU/UK standard), or an older NEDC (EU, phased out around 2018–2020) figure — since these testing procedures produce different numbers for the identical vehicle, on top of the pure unit conversion. WLTP figures tend to be more realistic than the older NEDC cycle, which was criticized for producing unrealistically optimistic numbers under lab conditions.

For budgeting a road trip’s fuel cost, convert your car’s rating to L/100km, or gallons per mile if you prefer to stay in US units, and multiply directly by your planned distance and local fuel price — this avoids the awkward extra division step that mpg-based calculations require, since L/100km is already expressed as “fuel used per distance,” matching how you will actually be paying for gas.

A worked road-trip budgeting example

Suppose you are planning a 450 km road trip in a car rated at 6.5 L/100km, with fuel priced at 1.70 per liter in local currency. Multiply distance by consumption rate: 450 × (6.5 ÷ 100) = 29.25 liters needed, then multiply by price: 29.25 × 1.70 ≈ 49.73 in local currency for fuel — a calculation that stays entirely in metric units without ever needing to touch gallons or miles.

Doing the same estimate in US units for a 280-mile trip in a 36 mpg car at $3.50 per gallon: divide distance by mpg to get gallons needed, 280 ÷ 36 ≈ 7.78 gallons, then multiply by price, 7.78 × 3.50 ≈ $27.22. Both approaches work fine within their own unit system — the important part is not mixing L/100km distance with a gallon price, or mpg with a liter price, without converting first.