I'm an American buying a used car in the UK. The listing says 45 mpg. Is it as economical as a 45 mpg car back home?

No. 45 mpg (Imperial) × 0.832674 = 37.5 mpg (US). Still a very fuel-efficient car by US standards — 37.5 mpg is excellent for a non-hybrid — but it won't stretch fuel as far as a true 45 mpg (US) car. Always convert before calculating road trip costs.

Why do UK and US both call it 'mpg' if they mean different things?

Both measure miles per gallon — but 'gallon' has two legal definitions. The UK inherited the Imperial gallon from British weights and measures. The US defined its own gallon (based on the old wine gallon) at a different volume. Both are 'gallons' in everyday speech, which is exactly why the confusion persists.

I'm relocating from the UK to the US. My UK car's computer shows 48 mpg. What should I expect in American terms?

48 mpg (Imperial) × 0.832674 = 40 mpg (US). Your onboard computer is calibrated for Imperial gallons, so the displayed figure will still be 48 — but American drivers and fuel cost calculators will expect the US gallon equivalent of 40 mpg. Recalibrate your mental benchmark. See also: [mpg (Imperial) to L/100km](/fuel/mpg-imperial-to-l-per-100km/).

A friend says his car gets 50 mpg (Imperial). Another friend's car gets 42 mpg (US). Which is actually more efficient?

50 mpg (Imperial) × 0.832674 = 41.6 mpg (US). The US car at 42 mpg is very slightly more efficient. The two vehicles are almost identical in real-world fuel consumption — the difference is negligible in practice.

MPG (Imperial) to MPG (US) Converter

Enter a value to instantly convert between fuel economy units.

From

Precision4 dp

1 MPG (Imperial) = 0.8327 MPG (US)

Key Formulas

MPG (US) → Liters per 100km

L/100km = mpg × 235.215

Liters per 100km → MPG (US)

mpg = L/100km × 235.215

Km per Liter → MPG (US)

mpg = km/L × 2.35215

MPG (US) → Km per Liter

km/L = mpg × 0.425144

Formula

mpg (US) = mpg (Imperial) × 0.832674

Both the UK and USA use 'mpg' to rate fuel economy, but they're measuring different gallons. The Imperial gallon (4.546 litres, used in the UK) is roughly 20% larger than the US gallon (3.785 litres). Because the UK gallon is bigger, any car will achieve a higher mpg figure in Imperial units than in US units — even though its actual fuel consumption hasn't changed. A UK car claiming 40 mpg (Imperial) is only achieving 33.3 mpg by American measurement. This confusion trips up car buyers, expats, and automotive journalists constantly.

Source: NIST SP 811, Table B.8

Last reviewed: 2026-06-11 · see our methodology

Frequently Asked Questions

Real-World Examples

A UK car rated 40 mpg (Imperial) is actually 33.3 mpg (US) — still strong by American standards but significantly lower than the headline UK number suggests.

40 mpg imp = 33.31 mpg

A UK turbodiesel at 55 mpg (Imperial) equals 45.8 mpg (US) — genuinely excellent, comparable to the best non-hybrid petrol cars in the American market.

55 mpg imp = 45.8 mpg

A typical UK family estate at 30 mpg (Imperial) delivers 25 mpg (US), roughly average for a mid-size sedan in the United States.

30 mpg imp = 24.98 mpg

A UK plug-in hybrid claiming 65 mpg (Imperial) on the official test converts to 54.1 mpg (US) — impressive even by US hybrid standards.

65 mpg imp = 54.12 mpg

mpg to l per 100km l per 100km to mpg kml to mpg mpg to kml

MPG (Imperial) to MPG (US) Converter

Key Formulas

Formula

Frequently Asked Questions

Real-World Examples

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