About the Fuel Economy Converter

The fuel economy converter translates between miles per gallon (US), miles per gallon (UK), kilometres per litre, and litres per 100 kilometres. Whether you are comparing a car's fuel economy from an American review (mpg US), a British review (mpg UK), a European specification (L/100km), or an Asian specification (km/L), this tool gives you an instant conversion in either direction.

Fuel economy is unusual among unit conversions because some units are reciprocals of each other. Miles per gallon, kilometres per litre, and miles per UK gallon are all "distance per volume" units, where a higher number means better fuel economy. Litres per 100 kilometres is a "volume per distance" unit, where a lower number means better fuel economy. Converting between these two families requires taking a reciprocal, not just multiplying by a factor.

A further complication is the difference between the US gallon (about 3.785 litres) and the UK imperial gallon (about 4.546 litres). A car that gets 30 mpg US gets about 36 mpg UK, because the UK gallon is larger. This converter handles both explicitly to avoid the common mistake of treating them as interchangeable.

How It Works

The converter uses kilometres per litre as its internal base unit. Every input is first converted to km/L, then converted from km/L to the target unit:

1 mpg US  = 0.425144 km/L    (1 mile / 1 US gallon)
1 mpg UK  = 0.354006 km/L    (1 mile / 1 UK gallon)
1 km/L    = 1 km/L
1 L/100km = 100 / (km/L)     (reciprocal relationship)

Conversion formulas:
mpgUS -> kmL :  kml = mpgUS * 0.425144
mpgUK -> kmL :  kml = mpgUK * 0.354006
L100 -> kmL  :  kml = 100 / L100

kmL -> mpgUS :  mpgUS = kml / 0.425144
kmL -> mpgUK :  mpgUK = kml / 0.354006
kmL -> L100  :  L100  = 100 / kml

The factor 0.425144 comes from dividing the mile (1.609344 km) by the US gallon (3.785411784 L), giving 0.425144 km/L per mpg US. The factor 0.354006 comes from dividing the mile by the UK gallon (4.54609 L), giving 0.354006 km/L per mpg UK. The two factors differ because the US and UK gallons differ by about 20%.

The litres per 100 km unit is a reciprocal: it measures volume per distance rather than distance per volume. Converting between km/L and L/100km requires the formula L/100km = 100 / (km/L), which is a division, not a multiplication. This is why a car with 5 L/100km fuel economy has a higher km/L value (20 km/L) than a car with 10 L/100km (10 km/L), even though the first number is smaller.

Worked Examples

To convert 30 mpg US to L/100km, the calculation is: 30 mpg US = 30 * 0.425144 = 12.75 km/L, then 100 / 12.75 = 7.84 L/100km. So a car that gets 30 mpg US uses about 7.84 litres of fuel per 100 kilometres. This is a typical fuel economy for a mid-size car.

To convert 7 L/100km to mpg US, the calculation is: 7 L/100km = 100 / 7 = 14.29 km/L, then 14.29 / 0.425144 = 33.60 mpg US. So a car that uses 7 litres per 100 kilometres gets about 34 miles per US gallon, which is why European and American fuel economy figures appear quite different at first glance.

To convert 40 mpg UK to mpg US, the calculation is: 40 mpg UK = 40 * 0.354006 = 14.16 km/L, then 14.16 / 0.425144 = 33.31 mpg US. So a car that gets 40 mpg UK gets about 33 mpg US, because the UK gallon is larger. This is why British fuel economy figures look better than American ones for the same car.

When to Use This Tool

Use the fuel economy converter whenever you need to translate between fuel economy units:

Comparing car specifications from reviews in different countries.
Converting between US mpg, UK mpg, km/L, and L/100km when car shopping.
Interpreting fuel economy figures on international rental cars.
Calculating the cost of a road trip with fuel priced in a different unit.
Comparing the efficiency of hybrid or electric vehicles across markets.
Working with logistics or fleet management data from multiple regions.
Teaching the reciprocal relationship between distance-per-volume and volume-per-distance units.

Limitations & Disclaimer

This converter handles fuel economy for internal combustion vehicles only. It does not convert electric vehicle efficiency units (miles per kWh, km per kWh, Wh per km) or energy density units. The US and UK gallon distinction is enforced; treating them as interchangeable produces errors of about 20%. For combined fuel economy (city plus highway weighting), use the manufacturer's combined figure directly. See our disclaimer for full details.

Frequently Asked Questions

What is the difference between US mpg and UK mpg?

The US gallon is approximately 3.785 litres, while the UK imperial gallon is approximately 4.546 litres. A car that travels 30 miles on one US gallon travels 30 miles on 3.785 litres, while a car that travels 30 miles on one UK gallon travels 30 miles on 4.546 litres. The UK gallon is larger, so the same fuel efficiency gives a higher mpg UK number than mpg US.

Why is L/100km the inverse of km/L?

Because they measure complementary quantities. Km/L measures distance per volume (how far you go on a litre), while L/100km measures volume per distance (how much fuel you use over 100 km). A higher km/L means better economy, but a higher L/100km means worse economy. Converting between them requires the formula L/100km = 100 / (km/L), a reciprocal rather than a simple multiplication.

Which unit should I use to compare cars?

It depends on your country. The United States uses mpg US, the United Kingdom uses mpg UK, most of Europe and many Asian countries use L/100km, and some Asian and Latin American countries use km/L. To compare cars from different markets, convert all figures to the same unit first using this converter.

Why do European cars seem less efficient than American cars?

They do not, but the units can be misleading. A car with 7 L/100km fuel economy sounds worse than 30 mpg, but they are roughly equivalent (7 L/100km is about 33.6 mpg US). The L/100km unit gives lower numbers for more efficient cars, while the mpg unit gives higher numbers for more efficient cars, which can confuse direct comparison.

How accurate are the conversions?

The conversion factors are exact, derived from the exact definitions of the mile, the US gallon, and the UK gallon. The only source of error is floating-point rounding, which is negligible. The converter displays up to six decimal places, which is more than enough for any practical comparison.

Does this converter handle electric vehicle efficiency?

Not directly. Electric vehicles use units such as miles per kWh, kilometres per kWh, or watt-hours per kilometre, which this converter does not handle. The principle is the same (distance per energy versus energy per distance), but the units are different. Use a dedicated EV efficiency converter for those.

Last updated: July 21, 2026 · Author: HT99 Tools Editorial Team · Reviewed by: HT99 Tools Editorial Team