Fuel Economy Converter
Convert fuel economy between US mpg, UK mpg, km/L and L/100km — correctly handling the inverse relationship.
How it works
km/L = mpg × (1.609344 ÷ gallon-litres) · L/100km = 100 ÷ (km/L) (US gallon = 3.785412 L, UK gallon = 4.54609 L)
Fuel economy comes in two opposite flavours: distance-per-fuel (mpg, km/L — higher is better) and fuel-per-distance (L/100km — lower is better), so you cannot convert with a single multiply. This tool routes everything through km per litre, then inverts to L/100km with 100 ÷ (km/L). Note US and UK gallons differ (3.785 L vs 4.546 L), so US mpg and UK mpg are not the same number.
Worked example
Convert 30 US mpg to L/100km: 30 × (1.609344 ÷ 3.785412) = 12.754 km/L, then 100 ÷ 12.754 ≈ 7.84 L/100km. So a 30 mpg car uses about 7.8 litres every 100 km.
Frequently asked questions
Why can’t I just multiply mpg to get L/100km?
Because they measure opposite things — miles per gallon rises as efficiency improves, while litres per 100 km falls. The relationship is inverse: you convert to a common km/L first, then take 100 ÷ (km/L). The product of mpg and L/100km is roughly constant (about 235 for US mpg).
Are US and UK mpg the same?
No. A UK (imperial) gallon is 4.546 L versus the US gallon’s 3.785 L, so the same car scores about 20% higher in UK mpg. Always check which gallon a figure uses before comparing cars.
What is a good L/100km figure?
Lower is better. A modern efficient petrol car sits around 5–7 L/100km, while larger vehicles use 9–12 or more. That corresponds roughly to 34–47 US mpg for the efficient range.
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This calculator is for educational purposes. Double-check important results before acting on them.