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MPG vs L/100km: How to Compare Fuel Economy
Half the world measures fuel economy in miles per gallon, the other half in litres per 100 kilometres. They are not just different units — they move in opposite directions, which makes mental comparison surprisingly tricky.
The key difference: higher vs lower is better
MPG measures distance per unit of fuel — higher is better. L/100km measures fuel per unit of distance — lower is better. This inversion is why you cannot compare the two numbers directly: 30 MPG and 8 L/100km look unrelated, yet they describe nearly the same car.
There is also a US/UK split: the US gallon is 3.785 litres, the imperial gallon is 4.546 litres. A car rated at 40 MPG (US) is 48 MPG (UK) — same car, different gallon. Always check which gallon a figure uses.
The exact conversion
For US gallons: L/100km = 235.21 ÷ MPG, and MPG = 235.21 ÷ L/100km. For imperial gallons the constant is 282.48. So 35 MPG (US) = 235.21 ÷ 35 ≈ 6.72 L/100km, and 5 L/100km = 235.21 ÷ 5 ≈ 47 MPG (US).
The Fuel Economy Calculator converts in both directions instantly and also computes fuel use per month or year from your driving distance.
From economy to money
Fuel economy only matters once it becomes a cost. The chain is simple: fuel needed = distance × (L/100km ÷ 100), then cost = fuel × price per litre. A 500 km trip in a car using 7 L/100km at $1.60 per litre needs 35 litres and costs $56.
The Fuel Cost Calculator runs this for any distance, and the Trip Cost Calculator adds passengers to split the bill. Comparing two cars? A drop from 9 to 6.5 L/100km over 20,000 km a year saves 500 litres — roughly $800 a year at $1.60 per litre.
Why your real numbers differ from the brochure
Official ratings come from lab cycles. Real consumption is typically 10–25% higher: cold starts, short trips, roof racks, underinflated tires and highway speeds above 110 km/h all add fuel. Winter driving alone can add 10–15%.
Measure your own economy instead: fill the tank completely, reset the trip meter, drive normally, refill and divide litres by kilometres × 100. Three tanks give a reliable personal figure — use it, not the brochure number, in cost calculations.