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

No — 45 mpg (Imperial) × 0.832674 = 37.5 mpg (US). It's still a very fuel-efficient car by US standards (37.5 mpg would be excellent for a non-hybrid), but it won't stretch your fuel budget as far as 45 mpg (US) would. Always convert before calculating fuel costs.

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

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

My UK car's service book shows fuel consumption in mpg. I'm moving to the US — will American fuel stations affect my economy differently?

Yes. Your car's actual fuel consumption (liters per 100km) doesn't change, but you'll notice your US fuel economy reading is about 17% lower in mpg terms. If your onboard computer shows 50 mpg (calibrated for Imperial gallons), the American equivalent is 41.6 mpg. Recalibrate your mental benchmarks. See also: [mpg (Imperial) to L/100km](/fuel/mpg-imperial-to-l-per-100km/).

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

Comparing fuel economy between a UK car and an American one? The numbers look similar but they mean different things. Miles per Imperial gallon (used in the UK) and miles per US gallon (used in the USA) both say 'mpg' but use gallons of different sizes. The Imperial gallon is 4.546 liters; the US gallon is only 3.785 liters. Because the Imperial gallon is larger, a car will always show a higher mpg (Imperial) number than mpg (US) for identical real-world efficiency. A car doing 40 mpg in the UK is actually only achieving 33.3 mpg by American measurement — not 40. This confusion trips up car buyers, expats, and journalists constantly.

Source: NIST SP 811, Table B.8

Frequently Asked Questions

Convert the UK figure first: 50 mpg (Imperial) × 0.832674 = 41.6 mpg (US). So yes, you would be misled — the 50 mpg (Imperial) headline makes the UK car sound more efficient, but when converted to US units it's actually 41.6 mpg (US), which is slightly worse than your old car's 42 mpg (US). The larger Imperial gallon inflates the mpg figure by about 20%, which is exactly the kind of apples-to-oranges comparison that catches out buyers moving between markets.

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 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 but likely optimistic in real-world conditions.

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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