Same consumption in miles per UK gallon: 35.31
The mpg field is the US gallon. The UK gallon is only the note, so it is not mixed up with a US window sticker.
How it works
Fuel economy is quoted in three directions. Most of Europe and China use liters per 100 kilometers, where a larger number is worse. The United States uses miles per US gallon, where a larger number is better. Japan often uses kilometers per liter, which points the same way as mpg. This page converts those three. A fourth line shows miles per UK gallon so the imperial figure is not mistaken for the US one. You can also type the liters you just pumped and the kilometers on the trip, and the page fills L/100 km from that pair.
One international mile is 1.609344 km exactly. One US gallon is 231 cubic inches, which is 3.785411784 liters. Dividing 100 km by the mile, then multiplying by the gallon, gives the constant 235.214583333. US mpg = 235.214583333 ÷ (L/100 km). km/L = 100 ÷ (L/100 km). The inverse is the same constant: L/100 km = 235.214583333 ÷ mpg. The UK gallon used here is 4.54609 liters, so the imperial constant is 282.480936367. The UK line is a note. It is not one of the three fields you edit.
A fill-up, worked in both directions
Fifty liters over 600 km is (50 / 600) × 100 = 8.333… L/100 km, shown as 8.33. The same trip is 600 / 50 = 12 km/L exactly, shown as 12.00. US mpg is 235.214583333 / 8.333333333 = 28.22575, shown as 28.23. The UK note on that same consumption is 33.90 mpg. If a British review says “34 mpg” and an American window sticker says “28 mpg”, they can be describing nearly the same car.
Eight L/100 km is a cleaner checkpoint. km/L is 100 / 8 = 12.5 exactly. US mpg is 235.214583333 / 8 = 29.401822917, shown as 29.40. The UK note is 35.31 mpg. Going back from 29.40 mpg does not return exactly 8, because 29.40 is already rounded. The page stores the number you type. Type 8 in the L/100 km box when you want the forward example.
Why your trip computer disagrees
The formula assumes the liters and the kilometers describe the same fuel. A pump that stops at a round number, a tank that was not filled to the same point twice, and a few liters used while idling all move the result. Odometers and GPS distances differ by a percent or two. Winter fuel, tire pressure, and a roof box are real changes in consumption, not conversion errors. This page does not adjust for them.
There is no US “city” and “highway” blend, and no conversion from miles per liter. To compare a brochure, convert each published figure into L/100 km and compare those. Do not average mpg values and then convert: the average of 20 mpg and 40 mpg is not the mpg of the combined distance unless the distances match.
Worked examples
Input: 8 L/100 km
Output: 12.50 km/L and 29.40 mpg (US). The UK note reads 35.31 mpg
Input: 50 L pumped, 600 km driven
Output: 8.33 L/100 km, 12.00 km/L, 28.23 mpg (US)
Input: 30 mpg (US)
Output: 7.84 L/100 km and 12.75 km/L
Limits
- Edited fields show two decimal places. 30 mpg is 7.840486 L/100 km and is shown as 7.84. km/L on that row is 12.75.
- Zero and negative numbers are rejected. A car that used no fuel has an undefined L/100 km.
- The UK gallon line updates from the result. Typing in it is not possible, on purpose.
Common mistakes
- Reading a UK mpg sticker as if it were US mpg. At 8 L/100 km the two answers are 35.31 and 29.40.
- Dividing kilometers by liters and calling the result L/100 km. Kilometers per liter is the reciprocal path: 12 km/L is 8.33 L/100 km, not 12.
- Averaging two mpg numbers from unequal trips and then converting. Convert each leg, weight by distance, then combine.
Questions
Where does 235.215 come from?▾
Is the gallon the one at a US pump?▾
Does a full tank of 45 L and a 500 km highway run get a special formula?▾
Sources
- International mile: 1.609344 km exactly. US gallon: 231 in³ = 3.785411784 L.
- UK gallon: 4.54609 L. Imperial mpg uses that gallon and the same mile.