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.

Deriving the MPG ↔ L/100km bridge

The two units measure inverses of one another, so the derivation is worth writing out. Start from definitions: 1 US gallon = 3.785411784 litres exactly, and 1 mile = 1.609344 kilometres exactly (NIST SP 811, the US unit-system guide). MPG is miles per gallon — distance divided by fuel. L/100km is litres per 100 km — fuel divided by distance, with the “per 100 km” chosen so the typical numbers land in the 4-12 range. Combining the conversions:

L/100km = (3.785411784 / 1.609344) × (100 / MPG)
L/100km = 235.2145 / MPG

That single constant — 235.2145 — is the bridge most fuel- economy converters bake in. Going the other direction,MPG = 235.2145 / L/100km. For UK imperial-gallon MPG the constant is 282.481 because the imperial gallon is 4.54609 L (about 20.1% larger than the US gallon). Our converter uses the exact constants rather than the rounded forms to ensure round-trip accuracy through the km/L canonical anchor.

Sources & references

The mile-to-kilometre (1.609344, exact) and US-gallon-to-litre (3.785411784, exact) conversion factors are normative in NIST SP 811. The EPA standardised test cycles (FTP-75, HWFET, SC03, US06) and the WLTP cycle definition (UN ECE GTR 15) are the test procedures whose results window-sticker MPG and EU L/100km labels derive from. The MPG illusioncitation is Larrick & Soll, “The MPG Illusion,” Science, vol. 320 (2008), pp. 1593-1594 (DOI:10.1126/science.1154983). Full primary references in the Sources block below.

Assumptions & limitations

• US gallon by default.Unqualified MPG inputs assume the US liquid gallon (3.785411784 L). UK imperial MPG (4.54609 L per gallon) is supported as an explicit mode but isn’t auto-detected from the number itself.
• Petrol-engine equivalence assumed for unit arithmetic.Diesel, LPG, CNG, E85, and hydrogen all carry different energy densities. The unit conversion is fuel-agnostic, but the “real-world” cost interpretation requires fuel-specific pricing the converter doesn’t apply.
• No electric-vehicle MPGe handling. MPGe (1 gallon-equivalent = 33.7 kWh per EPA convention) would need a separate energy-equivalence path.
• Round-trip through km/L is lossy at extreme precision. Inputs with more than 6 significant figures may drift in the last digit through the canonical anchor — a non-issue for the 1-3 significant figures actually used in driving.
• No driving-style adjustment. The 15-30% gap between published and real-world figures is rough guidance, not a per-vehicle correction the converter applies.
• L/100km display rounds to 1 decimal. MPG ↔ L/100km round-trips that involve a value pinned to a single decimal will not always exactly equal the original input.

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.