How MPG converts to L/100 km — and why the math is reciprocal
Fuel economy is the one quantity where "higher is better" turns into "lower is better" depending on unit.
MPG (miles per gallon) measures distance per unit fuel. "Higher is better" — a car that gets 40 MPG uses less fuel than one that gets 20 MPG for the same journey.
L/100 km measures fuel per 100 distance. "Lower is better" — 5 L/100km beats 10 L/100km.
The two are reciprocals. Going from one to the other means a division by the value (not a multiply), plus a unit conversion on both sides.
MPG (US) → L/100 km:
\frac{L}{100\,km} = \frac{235.214}{MPG}
So 30 MPG = 7.84 L/100 km. 40 MPG = 5.88. 50 MPG = 4.70.
L/100 km → MPG (US):
MPG = \frac{235.214}{L/100\,km}
So 6 L/100km = 39.2 MPG. 8 L/100km = 29.4 MPG.
The 235.214 constant is (US-gallon-in-litres × 100) ÷ mile-in-km = (3.785 × 100) ÷ 1.609. For imperial (UK) gallons, use 282.481 instead because the UK gallon is bigger (4.546 L).
If you only remember one thing: the conversion is "235 over your number", not "multiply by 0.425". The latter is the trap.