L/100 km and MPG

Two ways to say the same trip

Litres per 100 km says how much fuel a distance uses. Miles per gallon says how far a volume of fuel goes. They move in opposite directions: a thirstier car has a higher L/100 km and a lower MPG. You cannot add the two numbers or treat 8 L/100 km as “8 MPG”.

The identity is mpg = (100 × litres per gallon) / (L/100 km × kilometres per mile). The international mile is 1.609344 km. Which gallon you pick changes the result. NIST SP 811 states the US-gallon case as: divide 235.215 by the miles-per-gallon figure to get litres per 100 km. 24 US MPG is about 9.8 L/100 km.

Do not mix the gallons

A US liquid gallon is 3.785411784 litres (231 cubic inches at the international inch). An imperial gallon is 4.54609 litres. The L/100 km to MPG page and the MPG to L/100 km page ask which gallon you mean and keep that choice in both directions. Entering 30 and hoping the page guesses US or UK will not happen.

A worked US case: 8 L/100 km. Litres per US gallon ÷ (8 × 1.609344 km) × 100 is about 29.4 US MPG. The same 8 L/100 km is about 35.3 imperial MPG, because the imperial gallon holds more litres. Quoting “about 30 MPG” without the gallon is ambiguous by that gap.

Cost is a different sum

Consumption does not contain a price. A 400 km trip at 8 L/100 km uses 32 litres. At 1.90 per litre that is 60.80, and about 0.15 per kilometre. Change the price and the cost changes; the L/100 km does not. The fuel cost calculator takes distance, L/100 km and the pump price you type. It does not look up a tariff, and it does not convert miles or gallons — do that first if the dashboard is in MPG. A European or Nordic brochure already prints L/100 km. A US window sticker prints MPG. Convert the sticker, then type the local pump price (euro or krona per litre). The converter does not know either price.