Fuel Economy Converter

About fuel economy units

Fuel economy describes how far a vehicle travels per unit of fuel consumed. It is expressed in two fundamentally different ways that are inversely related, not proportional: miles per gallon (mpg) in the United States and United Kingdom measures distance per volume — higher is better; liters per 100 km (L/100km) used in most of Europe and Australia measures volume per distance — lower is better. Because these are reciprocal quantities, you cannot convert between them with a simple multiplication factor; the formula involves dividing a constant by the other value. A further complication: the US gallon (3.785 L) and the Imperial gallon (4.546 L) differ by about 20%, so a car rated at 40 US mpg and one rated at 40 UK mpg are not equally efficient — the UK car travels farther on the same fuel.

History & origin

Miles per gallon became standard in the United States as automobiles mass-produced in the 1910s–1920s replaced horses. The Imperial mpg likewise appeared in Britain when motoring became widespread. The liter-per-100-km convention emerged in continental Europe alongside the metric system's dominance and was designed to make fuel costs immediately calculable: multiply L/100km by trip distance in hundreds of km to get liters needed. The US EPA began publishing standardised fuel-economy ratings for new cars in 1974 after the oil crisis made efficiency a public concern.

Tips

To convert US mpg to L/100km (or vice versa), use the constant 235.215 — not a simple multiplier. The formula is: L/100km = 235.215 ÷ mpg(US). This is because the relationship is a reciprocal: a car that goes farther on a liter uses fewer liters per 100 km. The second gotcha is the gallon definition: a car achieving 30 US mpg is only achieving about 25 UK mpg (30 × 3.785 / 4.546 ≈ 24.97), because the Imperial gallon is larger. Always specify which gallon when quoting mpg across regions.

Common questions

Why can't you just multiply mpg by a fixed number to get L/100km?

Because mpg and L/100km are reciprocal quantities: one measures distance per volume, the other measures volume per distance. Doubling mpg halves L/100km — a multiplicative relationship would predict otherwise. The correct conversion is L/100km = 235.215 ÷ mpg (for US gallons) or 282.481 ÷ mpg (for Imperial gallons).

What is the difference between US mpg and UK mpg?

The US gallon is 3.785 liters; the Imperial (UK) gallon is 4.546 liters — about 20% larger. So the same car achieves a higher number in UK mpg than US mpg, even though the underlying efficiency is identical. For example, 40 US mpg ≈ 33.2 UK mpg. Always check which gallon a fuel-economy figure uses before comparing cars from different markets.

Which is better: high mpg or low L/100km?

Both mean the same thing: the vehicle is efficient. High mpg is better because more miles per gallon means less fuel per mile. Low L/100km is better because fewer liters per 100 km means less fuel per kilometer. A very efficient car might achieve 60 US mpg ≈ 3.9 L/100km; a fuel-hungry SUV might return 15 US mpg ≈ 15.7 L/100km.