One unit measures distance; the other measures fuel
Miles per gallon asks how far the car travels on a fixed amount of fuel. Litres per 100 kilometres asks how much fuel the car uses over a fixed distance. That is why efficient cars have a high MPG number but a low L/100 km number.
The relationship is reciprocal rather than linear. Doubling MPG roughly halves fuel consumption per distance, so adding ten MPG does not represent the same saving at every starting point.
US and Imperial gallons are not interchangeable
An Imperial gallon is larger than a US gallon. A car quoted at 40 Imperial MPG is not achieving the same consumption as 40 US MPG. Always identify the gallon standard before converting.
European and many other markets use L/100 km, while the United States commonly uses US MPG. UK material may use Imperial MPG, sometimes alongside metric figures.
Compare cost with the same distance and fuel price
For a buying decision, convert both vehicles to one consumption unit, choose a realistic annual distance, and apply the same fuel price. That turns an abstract unit difference into an estimated annual fuel amount.
Real consumption changes with speed, weather, load, tyres, traffic, and driving style. Official ratings are useful for consistent comparison, not a promise of what every driver will see.
Practical checklist
- Identify US MPG or Imperial MPG.
- Remember that lower L/100 km means less fuel.
- Convert all vehicles to the same unit before comparing.
- Use the same distance and fuel price assumptions.
- Treat official ratings as comparison figures, not guarantees.