I'm a British expat buying a car in the US. The dealer says it gets 28 mpg. How does that compare to cars I know from home?

28 mpg (US) × 1.200950 = 33.6 mpg (Imperial). In UK terms, that's below average for a new family car — most modern petrol hatchbacks manage 40–50 mpg (Imperial). American cars are generally less fuel-efficient partly due to cheaper fuel prices and different driving habits.

American car reviews say mpg. UK reviews say mpg. Why are the numbers always higher in UK reviews?

Because UK reviews use Imperial gallons (4.546 L) and US reviews use US gallons (3.785 L). The same car will always appear 20% more efficient in UK figures. This catches people out constantly — a 'frugal' 40 mpg UK car is only 33 mpg by US measurement. See also: [mpg (US) to L/100km](/fuel/mpg-to-l-per-100km/).

I'm calculating road trip fuel costs for a hire car in the UK. The car is American and rated 30 mpg. What mpg (Imperial) should I use?

30 mpg (US) × 1.200950 = 36.0 mpg (Imperial). Use 36 mpg when calculating UK fuel costs in pence per mile. Remember UK fuel is priced per liter, so you'll want to convert that to a per-gallon cost: 1 Imperial gallon = 4.546 liters × current price per liter. For a different approach, try [mpg (US) to L/100km](/fuel/mpg-to-l-per-100km/).

Is the conversion factor always the same, or does it depend on the vehicle?

The conversion factor (1.200950) is always the same — it's purely the ratio of gallon sizes (4.546 ÷ 3.785). It doesn't depend on the vehicle, engine size, or fuel type. The only time the factor changes is if you're converting to/from km/L or L/100km instead.

MPG (US) to MPG (Imperial) Converter

Enter a value to instantly convert between fuel economy units.

From

Precision4 dp

1 MPG (US) = 1.2009 MPG (Imperial)

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 (Imperial) = mpg (US) × 1.200950

Looking at an American car's EPA fuel economy rating and want to compare it to UK vehicles? Miles per US gallon and miles per Imperial gallon look identical on paper — both just say 'mpg' — but they refer to gallons of different sizes. The US gallon holds 3.785 liters; the Imperial gallon holds 4.546 liters. Because the Imperial gallon is about 20% larger, the mpg (Imperial) figure for any car is always about 20% higher than its mpg (US) figure. A car rated 25 mpg by the EPA would be 30 mpg (Imperial) — a meaningful difference when comparing across markets or estimating UK fuel costs.

Source: NIST SP 811, Table B.8

Frequently Asked Questions

Real-World Examples

A typical American sedan rated 25 mpg (US) by the EPA becomes 30 mpg (Imperial) — mid-range by UK standards for a petrol family car.

25 mpg = 30.02 mpg imp

An efficient US compact at 35 mpg (US) converts to 42 mpg (Imperial), which would be considered strong fuel economy in the UK market.

35 mpg = 42.03 mpg imp

A US pickup truck at 15 mpg (US) is only 18 mpg (Imperial) — well below average for the UK, where such trucks are uncommon partly due to fuel costs.

15 mpg = 18.01 mpg imp

A US hybrid achieving 50 mpg (US) hits 60 mpg (Imperial) — a headline-worthy number in any market and competitive with the best UK diesels.

50 mpg = 60.05 mpg imp

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

MPG (US) to MPG (Imperial) Converter

Key Formulas

Formula

Frequently Asked Questions

Real-World Examples

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