Three ways to describe fuel economy
- L/100 km (litres per 100 kilometres): how much fuel you use. Lower is better. Used in Europe, Australia, Canada and much of the world.
- km/L (kilometres per litre): how far one litre takes you. Higher is better. Common in Asia and Latin America.
- mpg (miles per gallon): how far one gallon takes you. Higher is better. Used in the US (US gallon) and the UK (imperial gallon).
L/100 km = litres ÷ km × 100
km/L = km ÷ litres = 100 ÷ (L/100 km)
mpg (US) = 235.215 ÷ (L/100 km)
mpg (UK) = 282.481 ÷ (L/100 km)
How to measure your real fuel economy
- Fill the tank completely, until the pump clicks off, and reset the trip meter.
- Drive normally until the tank is at least half empty, ideally longer.
- Fill up again to the same click-off point. The amount you put in is the fuel you used.
- Divide using the formulas above, or enter both numbers in the calculator.
Repeat over two or three tanks and average the results. One tank can be distorted by a single long motorway drive or a week of short trips.
Worked example
You drove 450 km and refilled with 36 litres.
- L/100 km = 36 ÷ 450 × 100 = 8.0
- km/L = 450 ÷ 36 = 12.5
- mpg (US) = 235.215 ÷ 8.0 = 29.4
- mpg (UK) = 282.481 ÷ 8.0 = 35.3
Going the other way, a car rated at 30 US mpg uses about 7.84 L/100 km, which is 12.75 km/L or 36.0 UK mpg.
Why mpg can be misleading
Because mpg measures distance per fuel, equal improvements in mpg do not save equal amounts of fuel. Going from 10 to 20 mpg saves 5 gallons every 100 miles; going from 30 to 40 mpg saves only 0.83 gallons over the same distance. This "mpg illusion" is one reason most countries prefer L/100 km, which shows fuel used directly. When comparing cars, compare L/100 km, or convert mpg to fuel used per 100 miles.
Typical values
| Vehicle (typical real-world) | L/100 km | mpg (US) |
|---|---|---|
| Small scooter or motorcycle | 2–4 | 59–118 |
| Small petrol car | 5–7 | 34–47 |
| Family car / hybrid | 4–8 | 29–59 |
| Large SUV or pickup | 10–15 | 16–24 |
These ranges are only a guide; your engine, driving style, traffic and climate make a big difference.
Tips to improve fuel economy
- Keep tyres at the recommended pressure.
- Accelerate smoothly and anticipate traffic so you brake less.
- Keep motorway speeds moderate; drag rises steeply with speed.
- Remove roof boxes and unnecessary weight when not needed.
- Service the vehicle on schedule: clean air filters and correctly gapped spark plugs help older engines.
Common mistakes
- Using the trip computer only. Many dashboards read a few percent optimistic. The fill-up method is the real figure.
- Not filling to the same level each time. Always fill until the first click-off.
- Mixing US and UK gallons. They differ by about 20%.
Cost per 100 km
Multiply your consumption by the fuel price to see what every 100 km costs. At a price of 1.80 per litre:
| Consumption | Fuel cost per 100 km |
|---|---|
| 5 L/100 km | 9.00 |
| 7 L/100 km | 12.60 |
| 9 L/100 km | 16.20 |
CO₂ emissions from your fuel figure
Burning fuel releases a fixed amount of carbon dioxide per litre. Using US EPA figures of 8,887 g of CO₂ per US gallon of petrol (gasoline) and 10,180 g per gallon of diesel, that is about 2.35 kg per litre of petrol and 2.69 kg per litre of diesel. A petrol car using 7 L/100 km therefore emits about 16.4 kg of CO₂ every 100 km from its exhaust.
Electric vehicles
Electric cars state consumption in kWh per 100 km (or miles per kWh). The cost works the same way: kWh/100 km × electricity price. A car using 16 kWh/100 km at 0.25 per kWh costs 4.00 per 100 km in electricity, although public fast-charging prices can be much higher than home rates.
Frequently asked questions
How do I convert mpg to L/100 km?
Divide 235.215 by US mpg, or 282.481 by UK mpg. For example 30 US mpg is 7.84 L/100 km.
How do I convert L/100 km to km/L?
Divide 100 by the L/100 km value. 8 L/100 km is 12.5 km/L.
Is a higher L/100 km better?
No, lower is better: it means the vehicle uses less fuel to travel 100 km. For km/L and mpg, higher is better.
Why is UK mpg higher than US mpg for the same car?
A UK gallon (4.546 L) is about 20% larger than a US gallon (3.785 L), so the car travels further per UK gallon.
How much CO₂ does a litre of petrol produce?
About 2.35 kg, based on the US EPA figure of 8,887 g per gallon. Diesel produces about 2.69 kg per litre.
How do I compare petrol and electric running costs?
Multiply L/100 km by the fuel price and kWh/100 km by the electricity price, then compare the cost per 100 km.
Sources
- NIST Handbook 44, Appendix C: definitions of the US gallon (3.785411784 L) and imperial gallon (4.54609 L); 1 mile = 1.609344 km.
- Larrick RP, Soll JB. The MPG Illusion. Science, 2008; 320: 1593–1594.
- US Department of Energy, fueleconomy.gov: Driving more efficiently.
Last reviewed 5 October 2026. Results are estimates; see our disclaimer.