MPG vs L/100km: Reading Fuel Like a Local

By Hutu Laurentiu

5 min read · Updated October 2026

Fuel economy is the rare conversion where bigger isn't just different — it's the opposite direction. The US asks "how far on a gallon?" (bigger = better). Europe asks "how much fuel for 100 km?" (smaller = better). Misread the direction and a gas-guzzler looks green.

The formula

L/100km = 235.2 ÷ MPG (US)

That's for US gallons. For UK mpg (imperial gallon = 4.55 L, not 3.79 L) use 282.5 ÷ MPG. A "40 mpg" car is 5.9 L/100km in the US but 7.1 L/100km in the UK — the single most common cross-border mistake in car ads.

The table for car shopping

• 20 mpg (US) = 11.8 L/100km — big SUV, thirsty
• 25 mpg = 9.4 L/100km — large crossover
• 30 mpg = 7.8 L/100km — solid family car
• 35 mpg = 6.7 L/100km — efficient hatchback
• 40 mpg = 5.9 L/100km — small economy car
• 50 mpg = 4.7 L/100km — hybrid territory
• 60 mpg = 3.9 L/100km — UK-style diesel/hybrid figures

Rule of thumb without math: US mpg ÷ 2, then a bit less gives L/100km. 40 mpg → 20 → ~6. Good enough to compare cars; use the formula to budget fuel costs.

Why Europe flipped the fraction

Miles per gallon is a distance per volume ratio; litres per 100 km is volume per distance. When you double distance, mpg doubles but L/100km halves. Consuming "fuel per fixed trip" maps better onto monthly budgets — that's why most of the world (and even US electric-car efficiency in mi/kWh debates) leans toward consumption units. Improvements also read more honestly: going 25→30 mpg saves 0.9 L/100km, while 35→40 saves only 0.8 — the same "5 mpg" step matters less at the high end.

Real cost, worked example

12,000 km/year, fuel at €1.80/L:

• 9 L/100km → 1,080 L → €1,944/year
• 6 L/100km → 720 L → €1,296/year

The 3 L/100km gap is €648/year — which is why the L/100km unit, awkward as it feels to Americans, makes running costs visible immediately.

The gallon trap, once and for all

Three gallons exist: US liquid (3.785 L), UK imperial (4.546 L), and the rare US dry gallon. Car specs never say which — but US cars mean US gallons, UK/Empire-market cars mean imperial. Canadian sources switched to litres decades ago. If a classifieds ad says "60 mpg" and was written in London, that's 4.7 L/100km, not the 3.9 a US reader assumes — a 20% difference.