Fuel Economy Converter (MPG ↔ L/100km ↔ km/L)
US MPG, UK MPG, L/100km and km/L on one page — converted exactly, with the gallon trap and the reciprocal illusion explained.
Two traps: the UK gallon is 20% bigger than the US gallon (so UK MPG numbers always look better), and MPG↔L/100km is a reciprocal — equal-looking MPG gains are NOT equal fuel savings. 13→14 MPG saves more fuel than 40→45.
Formula
⚠️ Estimates for planning and education — verify against manufacturer data and measured results. Performance figures are not a substitute for safe, legal driving.
US MPG, UK MPG, L/100km and km/L on one page — converted exactly, with the gallon trap and the reciprocal illusion explained.
About Fuel Economy Converter (MPG ↔ L/100km ↔ km/L)
A Brit, an American and a German compare cars: 50 MPG, 42 MPG and 5.6 L/100km — and all three are describing the same vehicle. Fuel economy is the unit-conversion minefield of the car world, complicated by two different gallons and a reciprocal relationship that makes percentage comparisons lie. This converter translates exactly between US MPG, imperial MPG, litres-per-100km and kilometres-per-litre, and explains why a 1-MPG improvement means completely different things at different ends of the scale.
How to use Fuel Economy Converter (MPG ↔ L/100km ↔ km/L)
- 1Enter — sensible defaults are pre-filled so you see a worked result immediately.
- 2Read the live results: .
- 3Check the "With your numbers" line to see the formula km/L = MPG(US) × 1.609344 / 3.785412; L/100km = 100 ÷ km/L; UK gallon = 4.546 L (US = 3.785 L) substituted step by step.
- 4Adjust inputs (or flip the unit toggle) until the scenario matches yours, then copy or share the result.
Why use Fuel Economy Converter (MPG ↔ L/100km ↔ km/L)?
- ✓Instant, free and private — every calculation runs in your browser, nothing is uploaded
- ✓Built on the published formula km/L = MPG(US) × 1.609344 / 3.785412; L/100km = 100 ÷ km/L; UK gallon = 4.546 L (US = 3.785 L) with sources cited on the page
- ✓Two traps: the UK gallon is 20% bigger than the US gallon (so UK MPG numbers always look better), and MPG↔L/100km is a reciprocal — equal-looking MPG gains are NOT equal fuel savings. 13→14 MPG saves more fuel than 40→45.
- ✓Switch units, tweak any input and watch every result update live
Frequently asked questions
Why do UK MPG figures always look better than US ones?+
The imperial gallon is 4.546 litres against the US gallon's 3.785 — 20.1% bigger — so the same car travels 20% more miles per UK gallon by definition. A US 35-MPG car is a UK 42-MPG car with no engineering difference whatsoever. Cross-Atlantic car reviews trip on this constantly; convert both to L/100km before believing any comparison.
Why do engineers prefer L/100km over MPG?+
Because consumption-per-distance is linear in what you actually buy: if a car uses 8 L/100km, a 10,000-km year costs 800 litres — straight multiplication. MPG is the reciprocal, which warps deltas: improving 13→14 MPG saves the same fuel as 40→50 MPG over equal miles (work it through — it's true). The 'MPG illusion' is documented in behavioral-economics literature: people consistently misrank savings when quoted in MPG.
What's a 'good' figure in each system?+
Very roughly, for combined driving: an efficient hybrid runs 4.5–5.5 L/100km (≈ 43–52 US MPG); a typical modern sedan 6.5–8 (29–36 MPG); a full-size truck 11–14 (17–21 MPG). In km/L — common in India, Japan and Brazil — those bands read 18–22, 12.5–15.5, and 7–9 km/L. Official cycles (EPA, WLTP, ARAI) each flatter differently, so same-cycle comparison matters more than the unit.
How do I convert a fuel-cost-per-mile from these numbers?+
Cost per distance = price per unit fuel ÷ economy in matching units: $3.60/gal at 30 US MPG = 12¢/mile; ₹105/L at 15 km/L = ₹7/km; €1.80/L at 7 L/100km = €12.60 per 100 km (the L/100km form needs only multiplication — another reason engineers like it). Our fill-up calculator measures your real-world economy to feed this math, which routinely runs 10–20% worse than the window sticker.
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