MPG vs L/100km and Other Fuel Efficiency Units

US mpg and L/100km are related by a reciprocal and a constant that comes from the US gallon and the statute mile:

L/100km = 235.215 / (mpg_US)
mpg_US = 235.215 / (L/100km)

235.215 is 100 × 3.785411784 / 1.609344. It is not 235, and it is not 100.

Fuel converter implements that inverse. It runs in the browser; no account.

Why “times 2.35” is wrong

People remember 235 and then multiply: 30 mpg × 2.35 ≈ 70, which they write as 7.0 after moving a decimal, or as 70 L/100km if they do not. 30 US mpg is 7.84 L/100km, not 70 and not 12. Linear scaling is what you do to convert km/h to mph. It is not what you do here.

The quantity underneath US mpg is miles / US gallon. L/100km is litres / (0.01 km), i.e. volume per distance. Inverting a ratio flips which quantity is on top. That is the whole trick.

km/L (used on some dashboards in Asia and Latin America) is distance per volume again, like mpg. 1 km/L = 100 / (L/100km). 12 km/L = 8.33 L/100km. Convert km/L ↔ mpg with gallons and miles, not with the L/100km inverse twice.

US gallon versus Imperial gallon

A US mpg uses the 3.785411784 L gallon. A UK mpg uses the 4.54609 L Imperial gallon.

Same fuel burn, different gallon:

UK mpg ≈ US mpg × (4.54609 / 3.785411784) ≈ US mpg × 1.20095

The UK constant is 282.481 / (mpg_UK) = L/100km (100 × 4.54609 / 1.609344).

A car that does 30 US mpg does about 36.0 UK mpg and 7.84 L/100km. Listing “30 mpg” on a British forum without saying US or UK is a 20 percent lie. Listing 30 mpg and 7.8 L/100km together is consistent only if the mpg is US.

Volume converter will turn 1 US gal into 3.785 L. That does not produce an economy figure. You still need the miles or the kilometres. The volume page is for tank size and jerrycans; see area, volume, density if litres are the only problem.

One numeric check: 30 US mpg

30 US mpg → L/100km: 235.215 / 30 = 7.8405 L/100km.

30 US mpg → km/L: 1.609344 × 30 / 3.785411784 = 12.752 km/L (equivalently 100 / 7.8405).

30 US mpg → UK mpg: 30 × 1.20095 ≈ 36.03 mpg (Imp).

Check the inverse: 235.215 / 7.8405 = 30.00. If you used 282.481 with 30 by mistake (UK formula on a US number), you would report 9.42 L/100km - a thirstier car that does not exist.

Open fuel converter, enter 30, US mpg → L/100km. You want 7.84. If you see 12.75, you landed on km/L. If you see 36, you landed on UK mpg.

What “better” means on each scale

Going from 20 to 30 US mpg saves more fuel per mile than going from 40 to 50. Gallons per mile are 1/20, 1/30, 1/40, 1/50: the drop from 0.050 to 0.033 gal/mi is larger than 0.025 to 0.020. L/100km makes that obvious (11.76 → 7.84 versus 5.88 → 4.70). Advertising a jump from 50 to 55 mpg as if it matched 15 to 20 mpg is the linear-brain habit again.

WLTP, EPA, and NEDC cycle numbers are different tests. Converting units does not convert test procedures. A European 6.0 L/100km WLTP figure is 39.2 US mpg only as a unit change, not as an EPA sticker.

Speed limits are a different converter: speed. Fuel economy changes with speed, but 100 km/h is not a fuel unit.

What to paste into a spreadsheet

Write 7.84 L/100km or 30 mpg (US). A column titled “mpg” that contains 7.84 will be averaged with 30. The unit converters hub is the rest of the toolkit; this page is only the inverted pair.

If you needed litres and you opened this article because the tank says 12 gal, skip economy and use volume: 12 US gal = 45.42 L. That multiplication is linear. Economy is not.