US gallon vs Imperial gallon is why one journey can be printed as two miles-per-gallon figures. US miles per gallon and Imperial miles per gallon use the same international mile and different gallons. A vehicle that burns a given volume of fuel over a given distance therefore has two MPG numbers. The Imperial figure is higher because the Imperial gallon is larger. That is a change of unit, not a claim that one gallon is the more accurate measure of fuel.
The international mile used here is 1.609344 kilometres. The US gallon is 3.785411784 litres. The Imperial gallon is 4.54609 litres. Those are definitions, not rounded guesses. Divide the Imperial gallon by the US gallon and the ratio is 4.54609 ÷ 3.785411784. Multiply a US mpg figure by that ratio and you get the Imperial mpg for the same fuel use. Thirty US mpg is 36.028497765146 Imperial mpg. Nothing about the car changed.
Miles per gallon asks how many miles you travel on one gallon. If the gallon holds more litres, one gallon takes you farther, so the MPG number rises. Fuel consumption did not improve. The measuring cup got bigger. Publishing “30 mpg” without saying which gallon leaves the reader to guess a difference of about a fifth.
The other direction is the same ratio inverted. Imperial mpg times 3.785411784 ÷ 4.54609 returns US mpg. Label the source before you convert. An unlabeled “mpg” field cannot do that honestly.
Litres per 100 kilometres counts fuel used for a fixed distance. Kilometres per litre counts distance for a fixed volume of one litre. They run in opposite directions: a larger L/100 km means more fuel per distance, and a larger km/L means more distance per litre. Miles per gallon agrees in direction with kilometres per litre and disagrees with litres per 100 kilometres. A bigger MPG is not a bigger L/100 km.
The conversion passes through litres per kilometre so the reciprocal is not applied backwards. For US mpg, litres per kilometre is 3.785411784 divided by the mpg times 1.609344. For Imperial mpg, the 3.785411784 is replaced by 4.54609. Litres per 100 kilometres is that rate times 100. Kilometres per litre is 1 divided by that rate.
Thirty US mpg is 7.840486111111 litres per 100 kilometres, 12.754311222908 kilometres per litre, and 36.028497765146 Imperial mpg. Eight litres per 100 kilometres is 29.401822916667 US mpg, 35.310117041478 Imperial mpg, and 12.5 kilometres per litre. The 12.5 is 100 ÷ 8. It is exact in that direction. The MPG figures are not “about 30” in both gallon systems. They differ by the gallon ratio.
30 US mpg = 36.028497765146 Imperial mpg
30 US mpg = 7.840486111111 L/100 km
8 L/100 km = 12.5 km/LPutting a US window sticker next to a UK figure and subtracting the MPG numbers treats two units as one. A car at 30 US mpg and a car at 36 Imperial mpg can be the same consumption. The second number looks more economical only until the gallon is named.
Another slip is to prefer the larger number across unit systems. Thirty-six is larger than 7.8, and it is also a different question: miles per gallon versus litres per 100 kilometres. Rank vehicles only after they share a unit. The Fuel Economy Converter converts one typed figure among US mpg, Imperial mpg, L/100 km, and km/L. It does not look up a vehicle, a fuel price, or a trip cost.
Zero and negative inputs are rejected because several of these units divide by the value. There is no sensible miles-per-gallon at zero gallons, and a negative economy is not a unit this conversion defines.
Use it when you already have a fuel-economy number and you need another of the four units, with the gallons named. It will not tell you which gallon a publisher used if the page you read omitted the word. It will not say that the US gallon or the Imperial gallon is the better definition. They are two sizes. The arithmetic starts after you know which one you were given.
The Imperial gallon is 4.54609 litres and the US gallon is 3.785411784 litres. A larger gallon raises miles per gallon for the same fuel use.
No. They are two defined sizes. Accuracy starts after you know which gallon the figure used.
No. Litres per 100 kilometres count fuel per distance. A higher MPG corresponds to a lower L/100 km.
It is 36.028497765146 Imperial mpg and 7.840486111111 litres per 100 kilometres, using the mile of 1.609344 kilometres.