Fuel economy methodology

Fuel economy is one of the trickiest unit conversions in common use because the three popular units measure fundamentally different things:

• MPG (US) — miles per gallon. Distance per unit fuel.
• km/L — kilometres per litre. Distance per unit fuel.
• L/100km (Europe) — litres per 100 kilometres. Fuel per unit distance.

The first two are direct linear units. The third is the inverse. Going from 5 L/100km to 4 L/100km saves more fuel than going from 9 L/100km to 8 L/100km, even though both look like a “1 unit improvement.” Our converter handles this correctly; most online converters don’t.

The canonical anchor: km/L

Internally we store all values as km/L (an inversion-safe linear unit). The conversion formulas:

km/L = (MPG × 1.60934) / 3.78541
L/100km = 100 / (km/L)
MPG = (km/L × 3.78541) / 1.60934

Where 1.60934 km = 1 mile and 3.78541 L = 1 US gallon (the 1959 international yard and pound agreement plus the US liquid gallon definition).

UK gallon vs US gallon

“MPG” without further qualification is the US MPG (US gallon = 3.78541 L). The UK imperial gallon is 4.54609 L, so UK MPG numbers are roughly 20% higher for the same physical fuel economy. The converter defaults to US; a future enhancement will expose imperial-gallon mode for UK-spec MPG figures.

The MPG illusion

Doubling MPG halves fuel consumption — but only when comparing equivalent driving. Larks Frederick (2008, Science) documented the “MPG illusion”: people consistently rank MPG improvements wrong because the unit is the inverse of fuel cost.

• Upgrade 10 MPG → 20 MPG: saves 500 gallons over 10,000 miles.
• Upgrade 30 MPG → 40 MPG: saves only 83 gallons over the same 10,000 miles.

Both are “10 MPG improvements” but the first is 6× more valuable in fuel saved. The L/100km unit makes this immediately obvious — going 23.5 → 11.8 L/100km (the same physical improvement) is clearly bigger than 7.8 → 5.9 L/100km.

This is why European fuel-economy labelling uses L/100km. It maps directly to fuel cost, which is the number that actually matters.

Real-world vs published

Published MPG figures (EPA in the US, WLTP in Europe) are from standardised driving cycles that consistently over-state real-world fuel economy by 10-30% depending on the vehicle and driver. The converter doesn’t adjust for this — it converts whatever number you input. For a real-world estimate, subtract 15% from EPA figures or 20% from WLTP as a first approximation.