Why fuel-economy conversion is reciprocal, not linear
MPG and L/100km measure the same thing, but the conversion passes through an inverse. Here is why.
Almost every conversion on this site multiplies by a factor and maybe adds an offset. Fuel economy is the exception — it's reciprocal: the conversion divides by your number, not multiplies.
Two formulations, one quantity
- Distance per fuel — MPG, km/L. "Higher is better." - Fuel per distance — L/100km, GPM. "Lower is better."
The same number, different framing. Going from one to the other means dividing through the value, not multiplying.
The constants
The conversion goes through the SI: distance in km, volume in L.
- 1 US gallon = 3.785411784 L - 1 mile = 1.609344 km
So 1 mile per (US) gallon = 1.609344 km / 3.785411784 L = 0.42514 km/L. Inverting, 1 km/L = 2.35214583 MPG (US).
A faster formula:
\frac{L}{100\,km} = \frac{235.214583}{MPG}
235.214583 = (gallon-in-litres × 100) ÷ mile-in-km. For imperial (UK) gallons, use 282.481 because the UK gallon is bigger (4.546 L).
Sample values
- 30 MPG (US) = 7.84 L/100km - 40 MPG (US) = 5.88 L/100km - 50 MPG (US) = 4.70 L/100km - 6 L/100km = 39.2 MPG (US)
Sources of confusion
American car reviews cite MPG; European ones cite L/100km; UK reviews sometimes use imperial MPG (≈ 1.2 × US MPG). The site keeps all three as separate units so you can convert in any direction.