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 achieve 40–50 mpg (Imperial). American cars are generally less fuel-efficient, partly due to cheaper fuel prices and larger engine sizes.

UK car reviews always seem to report better fuel economy than American reviews of the same car. Why?

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. A 'frugal' 40 mpg UK car is only 33 mpg by US measurement — which catches people out constantly. See also: [mpg (US) to L/100km](/fuel/mpg-to-l-per-100km/).

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

30 mpg (US) × 1.200950 = 36.0 mpg (Imperial). Use 36 when entering figures into UK fuel cost estimators. Remember that UK fuel is priced per litre, so a full Imperial gallon costs 4.546 × (pence per litre) to fill.

Is the 20% difference a rule of thumb, or is the conversion exactly 1.200950?

The 20% figure is a useful rule of thumb; the exact factor is 1.200950 (ratio of Imperial to US gallon: 4.546 ÷ 3.785). For rough mental estimates: multiply by 1.2. For exact calculations: use 1.200950. The difference between the two is less than 0.1% — negligible in practice.

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 both say 'mpg' — but they use gallons of different sizes. The US gallon holds 3.785 litres; the Imperial gallon holds 4.546 litres. Because the Imperial gallon is about 20% larger, any car's mpg (Imperial) figure 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, estimating UK fuel costs, or understanding why UK car reviews always seem to report better economy than American ones.

Source: NIST SP 811, Table B.8

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

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 petrol car standards.

25 mpg = 30.02 mpg imp

An efficient US compact at 35 mpg (US) converts to 42 mpg (Imperial), which would be considered good 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 fuel costs make such vehicles very expensive to run.

15 mpg = 18.01 mpg imp

A US hybrid achieving 50 mpg (US) hits 60 mpg (Imperial) — a headline-worthy number in either market, 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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