Fuel Economy Converter: Tips & How-Tos

Best Practices

Lower L/100km means better fuel economy. L/100km is an inverse scale — a car that uses 5 L/100km is more efficient than one that uses 9 L/100km. Many people confuse this with MPG, where higher is better.
US MPG and UK MPG are not the same. The US gallon (3.785 L) is smaller than the UK imperial gallon (4.546 L). A car rated 35 MPG (US) would be rated approximately 42 MPG (UK) — a 20% difference from the same physical fuel consumption.
km/L is the inverse of L/100km. km/L and L/100km convey the same information in opposite directions. To convert: km/L = 100 ÷ L/100km, and L/100km = 100 ÷ km/L. Japan uses km/L; Europe and Australia use L/100km.
All three metrics describe the same physical reality. MPG, L/100km, and km/L are all measuring the same thing — how much fuel it takes to travel a given distance. Converting between them gives equivalent information, just in a different format.

Manufacturer ratings are tested under ideal conditions. Official fuel economy figures are measured on a rolling road in controlled conditions. Expect 10–20% worse economy in real city driving due to traffic, cold starts, air conditioning, and driving style.
City fuel economy is always lower than highway. Frequent stopping and starting wastes energy through braking and inefficient low-speed engine operation. Highway driving at steady speeds is more efficient for combustion engines.
Cold starts reduce fuel efficiency significantly. Engines use more fuel in the first few minutes until they reach operating temperature. Short trips (under 5 km) never warm the engine fully and therefore always use more fuel proportionally.
Extra weight and cargo reduce MPG. Every 100 kg of extra weight reduces fuel economy by roughly 1–2%. Roof racks, heavy luggage, and passengers all affect efficiency — remove roof boxes when not in use.
Tyre pressure affects fuel economy. Under-inflated tyres create more rolling resistance, increasing fuel consumption by 1–3%. Check pressure monthly and before long trips to maintain optimal economy.

Convert all figures to the same unit before comparing. Comparing 32 MPG (US) with 8.5 L/100km is meaningless without conversion. Always convert all vehicles to the same metric before ranking them.
Calculate annual fuel cost, not just economy. A vehicle with better MPG may cost more to run if you drive significantly fewer miles per year. Multiply fuel cost per mile by annual mileage for a meaningful cost comparison.
Factor in fuel price differences between countries. UK petrol costs roughly twice what US petrol costs per litre. A 30 MPG car may be much cheaper to run in the US than a 40 MPG car is in the UK at current prices.

Putting two vehicles on the same footing before making a buying decision.

Note the US car's MPG (US) rating and the European car's L/100km rating.
Open the Fuel Economy Converter.
Enter the US MPG figure, set From to 'MPG (US)' and To to 'L / 100km'.
Compare the resulting L/100km figure to the European car's rating — lower is better.
Repeat in reverse to see the European car's MPG equivalent.

Calculating what a vehicle will actually cost to run per year.

Estimate your annual mileage (e.g. 12,000 miles).
Enter the vehicle's MPG in the converter to find fuel consumption per mile (1 ÷ MPG).
Multiply miles per year by fuel cost per gallon, divided by MPG: (12,000 ÷ 32) × $3.50 = $1,313 per year.
Repeat for each vehicle you are comparing to find the annual fuel cost difference.

Understanding the fuel efficiency of a rental car rated in L/100km when you are used to MPG.

A common source of confusion when reading international car reviews.

The UK gallon = 4.546 litres; the US gallon = 3.785 litres.
UK MPG is measured using the larger gallon, so more miles fit into one 'gallon' of distance.
To convert UK MPG to US MPG, multiply by 0.8327.
A car rated 50 MPG (UK) is approximately 41.6 MPG (US) — the same physical fuel consumption.
Use the converter to check any specific value instantly.

Planning a long road trip and estimating fuel stops.

Find your vehicle's fuel tank capacity (in litres or gallons — check the owner's manual).
Note your real-world fuel economy in the relevant unit.
For L/100km: range = (tank capacity in litres ÷ L/100km) × 100.
For MPG: range = tank capacity in gallons × MPG.
Subtract 10–15% as a reserve to avoid running out of fuel.

Diesel ratings in L/100km are typically lower (better) but diesel costs more per litre.

Note the L/100km rating for both the petrol and diesel versions of the car you are considering.
Estimate your annual mileage.
Calculate annual fuel volume for each: annual litres = (mileage in km ÷ 100) × L/100km.
Multiply by the respective fuel prices to get annual costs.
Factor in any difference in purchase price or servicing costs before concluding which is cheaper overall.