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mpg and litres per 100 km

UK mpg, US mpg and litres per 100 km, with the gallon named.

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L/100km (UK gallon)7.06
UK mpg40
US mpg33.3

The same “40 mpg” is 7.06 L/100km if the gallon is the UK gallon (4.54609 litres) and 5.88 L/100km if it is the US gallon (3.785411784 litres). A US figure looks more economical because the gallon is smaller. Lower L/100km is the more economical car. Fuel sold in the UK and in Ireland is in litres. Official UK mpg uses the UK gallon.

The factor is exact. The figure on screen is rounded for reading.

How to use it

  1. Choose litres per 100 km, or miles per gallon. The Irish site opens on litres per 100 km.
  2. If you start from mpg, say whether that gallon is the UK gallon or the US gallon.
  3. Read both mpg figures when you start from litres per 100 km. The UK mpg is the lower number.

Two gallons, one road

A UK gallon is exactly 4.54609 litres. A US gallon is exactly 3.785411784 litres. Fuel in the UK and in Ireland is sold in litres, and official UK miles per gallon use the UK gallon. A sticker or a review written in the US uses the smaller gallon, so the mpg number is higher for the same car.

Litres per 100 km = (litres in the gallon ÷ (mpg × 1.609344)) × 100. Forty UK mpg is 7.06 L/100km. The same digits, 40, treated as US mpg, are 5.88 L/100km. The page shows both so the gallon is a choice you make, not a silent assumption. Lower litres per 100 km is the more economical figure, which is the opposite direction from mpg.

Which figure a driver actually has

A UK logbook test or a UK review’s mpg is the imperial gallon. An Irish owner’s manual often quotes litres per 100 km. A car imported from the US, or a video that says “miles per gallon” without a country, needs the US gallon checked before the number is compared with a UK one.

The conversion is steady speed on paper. Cold starts, a roof box and the difference between the lab test and the road all move the real consumption. The miles and kilometres page is the distance half of the same journey.

Questions

What is 40 UK mpg in litres per 100 km?

About 7.06 L/100km. The same 40 as a US mpg is about 5.88 L/100km.

Why is US mpg higher?

The US gallon contains fewer litres, so the car travels the same mile on a smaller gallon and the mpg number rises. The litres per 100 km stay the same.

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