How to Convert MPG to Litres per 100 km
Learn how to convert miles per gallon (MPG) to litres per 100 km, understand the difference between US and Imperial MPG, and compare fuel economy figures side by side.
Contents
To compare fuel economy figures from different regions, convert MPG to litres per 100 km using the formula 235.215 / MPG for US gallons and 282.481 / MPG for Imperial gallons. This reciprocal conversion makes lower values better and aligns with most of the world outside the US.
Understanding MPG and L/100 km
Miles per gallon (MPG) measures distance travelled per unit of fuel. A higher MPG indicates better fuel economy. Litres per 100 km (L/100 km) measures fuel consumed per distance; a lower figure is better. The two are reciprocals, so a 10% improvement in MPG does not equal a 10% reduction in L/100 km.
Why US MPG and UK MPG Are Different
The US gallon is smaller than the Imperial gallon used in the UK. One US gallon equals 0.832674 Imperial gallons. Consequently, a car rated at 40 US MPG is equivalent to about 48 UK MPG. Always check the testing standard and gallon type before comparing.
US-to-UK MPG and L/100 km Conversion
To convert US MPG to UK MPG, multiply by 1.20095. For L/100 km conversions, use the constants above. The table below shows common values.
| US MPG | UK MPG (approx.) | L/100 km (from US MPG) |
|---|---|---|
| 20 | 24 | 11.8 |
| 30 | 36 | 7.8 |
| 40 | 48 | 5.9 |
| 50 | 60 | 4.7 |
Practical Steps for Comparing Listings
- Identify the MPG type (US or Imperial) from the listing or region.
- Convert all figures to L/100 km using the appropriate constant.
- Note the testing standard (EPA, WLTP, NEDC) as results differ.
- Compare vehicles with similar body styles and usage patterns.
When Missing Information Matters
Absence of fuel economy data may be a red flag, but it is not always a drawback. For example, a classic car listing may lack official MPG because tests were not standardised. In such cases, seek owner-reported figures or estimate from engine size and weight.
Trade-offs and Personal Priorities
Diesel cars often have lower L/100 km than petrol, but higher emissions. Hybrids excel in city driving; their highway MPG may be similar to efficient petrol cars. Match the fuel type and driving pattern to your needs before ranking options.