MPG vs L/100 km: Why Fuel Economy Numbers Mislead (and How to Compare Cars)
In the United States, a thrifty car gets 40 miles per gallon. In Europe, the same car uses 5.9 litres per 100 kilometres. Not only are the units different, the two systems measure fuel economy from opposite directions, which makes them surprisingly easy to misread.
This guide shows how to convert between the two, why bigger MPG gains are not always bigger savings, and how to work out what a more efficient car actually saves you.
Two ways to measure the same thing
- Miles per gallon (MPG) measures distance per unit of fuel. Higher is better.
- Litres per 100 kilometres (L/100 km) measures fuel per unit of distance. Lower is better.
Because one is the reciprocal of the other, you cannot convert them with a simple multiplication factor. Instead you divide a constant by the value:
L/100 km = 235.215 ÷ MPG (US)
MPG (US) = 235.215 ÷ L/100 km
The constant 235.215 combines the conversions from miles to kilometres and from US gallons to litres. The UK uses a larger imperial gallon (4.546 L instead of 3.785 L), so UK MPG figures are about 20 % higher for the same car:
L/100 km = 282.481 ÷ MPG (UK)
Always check which gallon a figure uses before comparing. The MPG to L/100 km converter and the L/100 km to MPG converter handle both.
Quick conversion table
| MPG (US) | L/100 km | MPG (UK) |
|---|---|---|
| 15 | 15.7 | 18.0 |
| 20 | 11.8 | 24.0 |
| 25 | 9.4 | 30.0 |
| 30 | 7.8 | 36.0 |
| 40 | 5.9 | 48.0 |
| 50 | 4.7 | 60.0 |
The MPG illusion
Here is a question that most people get wrong. Which upgrade saves more fuel over the same distance?
- A: replacing a 10 MPG truck with a 20 MPG truck
- B: replacing a 30 MPG car with a 60 MPG car
Most people pick B, because the MPG figure goes up by 30 instead of 10. The correct answer is A. Look at the fuel needed to drive 100 miles:
| Vehicle | Fuel for 100 miles | Fuel saved |
|---|---|---|
| 10 MPG truck → 20 MPG truck | 10 gal → 5 gal | 5 gallons |
| 30 MPG car → 60 MPG car | 3.33 gal → 1.67 gal | 1.67 gallons |
Researchers Richard Larrick and Jack Soll described this effect in the journal Science in 2008 and called it the “MPG illusion”. Because MPG puts fuel in the denominator, improvements at the low end of the scale save far more fuel than the same improvement at the high end.
L/100 km does not have this problem. Fuel consumption is on top, so differences can be compared directly: going from 23.5 to 11.8 L/100 km saves 11.7 litres per 100 km, while going from 7.8 to 3.9 saves only 3.9 litres. The bigger number is the bigger saving, exactly as intuition expects.
That is one reason why several countries publish fuel use per distance, and why the US window sticker now also shows “gallons per 100 miles”.
How to calculate your real savings
To compare two cars, convert both to fuel per distance and multiply by how far you drive.
Example: you drive 15,000 km a year and are comparing a car that uses 7.5 L/100 km with one that uses 5.5 L/100 km. Fuel costs 1.80 per litre.
- Fuel saved per 100 km: 7.5 − 5.5 = 2.0 L
- Distance in hundreds of km: 15,000 ÷ 100 = 150
- Fuel saved per year: 2.0 × 150 = 300 L
- Money saved per year: 300 × 1.80 = 540
With MPG figures, convert to gallons per 100 miles first (100 ÷ MPG) and follow the same steps.
Why your real consumption differs from the label
Official figures come from standardised laboratory test cycles. Real-world consumption is usually higher because of:
- motorway speeds well above the test cycle’s average speed,
- short trips where the engine never reaches operating temperature,
- air conditioning, roof boxes, heavy loads and under-inflated tyres,
- cold weather, which hits electric and hybrid cars particularly hard.
To measure your own consumption, fill the tank, reset the trip meter, drive normally, fill up again and divide the litres added by the kilometres driven, multiplied by 100. Repeat over several tanks for a reliable average.
Electric cars
Electric vehicles are rated in energy per distance: kWh/100 km in Europe, or miles per kWh in the UK. The US uses MPGe (“miles per gallon equivalent”), which treats 33.7 kWh of electricity as equal to one gallon of petrol. The same rule applies: per-distance figures like kWh/100 km are easier to compare and to turn into running costs.
Key takeaways
- MPG is distance per fuel (higher is better); L/100 km is fuel per distance (lower is better).
- Convert with L/100 km = 235.215 ÷ MPG (US) or 282.481 ÷ MPG (UK).
- Improving an inefficient vehicle saves far more fuel than the same MPG gain on an efficient one.
- To compare running costs, always work in fuel per distance.
Use the fuel economy converter to switch between MPG (US and UK), L/100 km and km/L.