Fuel Economy Converter
Convert mpg (US), mpg (UK), km/L, and L/100km.
About the Fuel Economy Converter
The fuel economy converter translates between four common ways of measuring fuel consumption: miles per US gallon, miles per UK gallon, litres per 100 kilometres, and kilometres per litre. The four units differ in both the volume unit (US gallon vs UK gallon vs litre) and the direction of the ratio (distance per fuel vs fuel per distance).
The US gallon is defined as exactly 3.78541 litres, while the UK imperial gallon is 4.54609 litres. A UK gallon is therefore about 20 percent larger, which means a UK mpg figure is about 20 percent larger than a US mpg figure for the same vehicle. A 30 mpg US car is about 36 mpg UK, all else being equal.
The two conventions invert the ratio. The "miles per gallon" convention measures distance per fuel, so a higher number means better economy. The "litres per 100 kilometres" convention measures fuel per distance, so a lower number means better economy. The two are inversely related: 30 mpg US equals about 7.84 L/100km, and 5 L/100km equals about 47 mpg US.
The conversion factors are derived from the underlying length and volume definitions. One mile equals 1.609344 kilometres and one US gallon equals 3.78541 litres, so 1 mpg US = 1.609344 / 3.78541 = 0.425144 km/L. The UK gallon conversion uses 4.54609 litres, giving 1.609344 / 4.54609 = 0.354006 km/L.
How It Works
The converter uses km/L as the pivot, with two helper functions:
toKmL(v, from):
k <- v * 0.425144 (if from is mpgUS)
k <- v * 0.354006 (if from is mpgUK)
k <- 100 / v (if from is L100)
k <- v (if from is kmL)
fromKmL(k, to):
v <- k / 0.425144 (if to is mpgUS)
v <- k / 0.354006 (if to is mpgUK)
v <- 100 / k (if to is L100)
v <- k (if to is kmL)The key conversion factors are:
1 mpg US = 0.425144 km/L = 7.840 L/100km
1 mpg UK = 0.354006 km/L = 9.402 L/100km
1 L/100km = 100 km/L (inverted) = 235.215 mpg US
1 km/L = 2.35215 mpg US = 2.82481 mpg UKThe factor 0.425144 comes from 1.609344 (km per mile) divided by 3.78541 (litres per US gallon). The factor 0.354006 comes from 1.609344 divided by 4.54609 (litres per UK gallon). The factor 235.215 is 100 divided by 0.425144, which converts L/100km directly to mpg US.
The two conventions are inversely related, so converting from mpg to L/100km uses division, while converting from L/100km to mpg uses multiplication. This is why a 10 mpg US improvement (say from 20 to 30 mpg) saves more fuel than a 10 mpg improvement from 30 to 40 mpg: the L/100km values are 11.76, 7.84, and 5.88 respectively, so the first improvement saves 3.92 L/100km and the second saves only 1.96 L/100km.
Worked Examples
Converting 30 mpg US to L/100km: the km/L value is 30 * 0.425144 = 12.7543 km/L, and the L/100km value is 100 / 12.7543 = 7.840 L/100km. This is a typical fuel economy for a small passenger car.
Converting 30 mpg UK to mpg US: the km/L value is 30 * 0.354006 = 10.6202 km/L, and the mpg US value is 10.6202 / 0.425144 = 24.97 mpg US. The UK gallon is about 20 percent larger, so the same car rated at 30 mpg UK is only about 25 mpg US.
Converting 5 L/100km to mpg US: the km/L value is 100 / 5 = 20 km/L, and the mpg US value is 20 / 0.425144 = 47.04 mpg US. This is a typical fuel economy for a hybrid car. Notice that 5 L/100km is dramatically better than 30 mpg US (which is 7.84 L/100km), even though the raw numbers look similar.
When to Use This Tool
Use the fuel economy converter when you need to:
- Compare a US vehicle's fuel economy with a European vehicle's specifications.
- Translate a UK car magazine's road test figures into US mpg for an American reader.
- Convert a rental car's fuel economy from L/100km to mpg US for trip planning.
- Compute the cost of a road trip by converting fuel economy to local units.
- Compare the efficiency of two vehicles that report consumption in different units.
- Translate fuel-economy regulations between US CAFE standards and European CO2 limits.
- Verify that a fuel-economy figure quoted in a news article makes sense.
Limitations & Disclaimer
This converter uses exact conversion factors derived from SI and the 1959 International Yard and Pound Agreement. It does not handle electric vehicle efficiency (kWh per 100 km, miles per kWh), plug-in hybrid blended figures, or CO2 emissions per kilometre. The converter assumes the standardised test-cycle figures (WLTP, EPA, or NEDC); real-world economy varies with driving conditions. See our disclaimer for full details.
Frequently Asked Questions
Why are US and UK mpg different?
Because the US gallon is defined as exactly 3.78541 litres, while the UK imperial gallon is 4.54609 litres (about 20 percent larger). A car that does 30 mpg UK therefore does only about 25 mpg US, since the same distance is being divided by a larger volume of fuel. Always check which gallon is meant when reading a fuel economy figure.
Why do some countries use L/100km while others use mpg?
Most metricated countries use L/100km because it matches their volume (litre) and distance (kilometre) units. The US and UK use mpg because they still use miles and gallons for everyday measurements. The L/100km convention has the advantage of being linear in fuel consumption, which makes the cost of fuel directly proportional to the number.
Which is better: a higher mpg or a lower L/100km?
Both indicate better fuel economy. A higher mpg means more distance per unit of fuel, while a lower L/100km means less fuel per unit of distance. The two are inversely related: doubling mpg halves L/100km. The crossover point is at about 28.25 mpg US, which equals 8.4 L/100km.
Why does a 10 mpg improvement save more fuel at low mpg than at high mpg?
Because the L/100km scale is non-linear with respect to mpg. Going from 20 mpg to 30 mpg reduces fuel consumption from 11.76 to 7.84 L/100km (a saving of 3.92 L/100km), but going from 30 mpg to 40 mpg reduces it from 7.84 to 5.88 L/100km (a saving of only 1.96 L/100km). This is why fuel-economy regulations focus on the least-efficient vehicles.
How is fuel economy related to CO2 emissions?
Directly, since each litre of petrol burned releases about 2.31 kg of CO2 (and each litre of diesel about 2.68 kg). A car emitting 120 g CO2 per kilometre therefore consumes about 5.2 L/100km of petrol. European regulations express fuel economy in terms of CO2 emissions per kilometre, while US regulations use mpg.
Does the converter handle electric vehicle efficiency?
No. Electric vehicles are typically rated in kWh per 100 km or miles per kWh, which are not directly comparable to liquid-fuel economy figures. To compare an EV with a petrol car, you would need to factor in the carbon intensity of electricity generation and the refining efficiency of petrol, which is beyond this tool's scope.
Last updated: September 9, 2026 · Author: HT99 Tools Editorial Team