🛢️ Fuel Economy Calculator

Why US MPG and UK MPG differ

The gallon is defined differently in the US customary and UK imperial systems, so an identical fuel-economy figure produces two different MPG numbers depending on which gallon definition is used.

• Because the UK gallon holds more fuel than the US gallon, the same vehicle covers more miles per UK gallon than per US gallon for identical real-world fuel economy — the UK MPG figure will always be numerically higher.
• Real-world fuel economy calculated from an actual fill-to-fill trip typically differs from a manufacturer's official/laboratory-rated economy figure, due to driving style, terrain, load and conditions.

What does a fuel economy calculator do?

This calculator computes real-world fuel economy from a completed trip — the distance driven divided by the fuel consumed to cover it — rather than relying on a manufacturer's laboratory-rated figure. It then converts the result across the four units most commonly used worldwide: liters per 100 kilometers (used across most of Europe and other metric countries), kilometers per liter, and miles per US gallon and miles per UK (imperial) gallon.

The US and UK 'gallon' are not the same unit. A US gallon is defined as 3.785411784 liters, while the UK imperial gallon is defined as 4.54609 liters — about 20% larger. This means the same vehicle reports a materially higher miles-per-gallon figure under the UK definition than under the US definition for identical real-world fuel economy, which is a frequent source of confusion when comparing MPG figures quoted in US versus UK/Commonwealth sources.

How to use this fuel economy calculator

1. Reset your trip odometer at a full tank, then drive normally.
2. Refill the tank completely and note the distance traveled since the reset, in kilometers.
3. Note the amount of fuel needed to refill the tank, in liters.
4. Enter both values to see your real-world fuel economy in L/100km, km/L, US MPG and UK MPG.

The formula behind fuel economy conversion

Liters per 100 km equals fuel used divided by distance, multiplied by 100. Kilometers per liter is simply distance divided by fuel used. To convert to miles per gallon, kilometers per liter is first converted to miles per liter by dividing by 1.609344 (kilometers per mile), then multiplied by the liters-per-gallon figure for the relevant gallon definition (3.785411784 for US, 4.54609 for UK).

Worked example: a car that travels 520 km on 38 liters of fuel has a fuel economy of (38 ÷ 520) × 100 ≈ 7.31 L/100km, or 520 ÷ 38 ≈ 13.68 km/L. Converting to imperial units gives approximately 32.2 miles per US gallon and approximately 38.7 miles per UK gallon for the same real-world fuel use.

Common mistakes

• Comparing a US MPG figure directly against a UK MPG figure without converting, since the gallon sizes differ by about 20%.
• Not filling the tank completely at both the start and end of the measured trip, which introduces error into the fuel-used figure.
• Confusing L/100km with km/L — a lower L/100km number and a higher km/L number both indicate better fuel economy, but they are inverse relationships, not directly comparable magnitudes.
• Assuming a single fill-up trip is representative of long-term average fuel economy, when short trips, cold starts and terrain can skew a single measurement.

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Referensi

1. National Institute of Standards and Technology (NIST) — standard unit definitions for the US gallon (3.785411784 L) and the mile (1.609344 km).
2. UK Weights and Measures Act — definition of the imperial gallon (4.54609 L).