MPG vs L/100km: Two Ways to Measure the Same Tank

To convert MPG to L/100km, divide 235.215 by the MPG figure. A sedan rated at 32 MPG consumes 235.215 ÷ 32 = 7.35 L/100km. The formula runs backwards without changing: a European hatchback rated at 8.5 L/100km is getting 235.215 ÷ 8.5 = 27.7 MPG. One constant, one division, either direction.

L/100km = 235.215 ÷ US MPG — and, identically, US MPG = 235.215 ÷ L/100km

That symmetry is the tell: MPG and L/100km are not two scalings of the same measurement, they are reciprocals. MPG counts distance per unit of fuel — bigger is better. L/100km counts fuel per unit of distance — smaller is better. The United States rates cars the first way on every EPA window sticker; the EU, Canada, and Australia rate them the second way. The MPG calculator turns a single fill-up into both figures at once, plus three more we'll meet below — but the rest of this page is about what the reciprocal relationship does to your intuition, because it quietly breaks it.

Where 235.215 comes from

Nothing about the constant is arbitrary. A US gallon is exactly 3.785411784 liters and a mile is exactly 1.609344 kilometers — both fixed by definition rather than measurement under NIST Special Publication 811, the mile via the 1959 international yard-and-pound agreement. A car doing 1 MPG therefore burns 3.785411784 liters every 1.609344 km. Scale that to 100 km and you have the constant: 3.785411784 × 100 ÷ 1.609344 = 235.215 (more exactly, 235.214583). Every fuel economy conversion between the two systems runs through this one number, and if you'd rather handle the underlying factors yourself — miles to kilometers, gallons to liters — the unit converter carries the same NIST-exact values.

Three scales, one tank

A third rating shares the tank: kilometers per liter, the customary figure in Japan, India, and much of Africa. Unlike L/100km it points the same way as MPG (higher is better), because it is MPG rescaled rather than inverted: km/L = MPG × 0.4251437, that factor being 1.609344 ÷ 3.785411784.

The km/L column doubles as a free error check on any hand conversion. A 30 MPG car does 30 × 0.4251437 = 12.75 km/L, and 100 ÷ 12.75 = 7.84 — the same L/100km the constant produced by the direct route. When a conversion survives that round trip, it's right.

Read down the middle column and notice how unevenly it moves. Five MPG at the top of the table (15 → 20) is worth 15.68 − 11.76 = 3.92 L/100km; a full ten MPG at the bottom (40 → 50) is worth only 5.88 − 4.70 = 1.18. That compression is not a rounding artifact — it is the whole story of the section after next.

Which gallon? The 20 percent trap

Britain quotes MPG too, against a different gallon. The imperial gallon is exactly 4.54609 liters — fixed by the UK Weights and Measures Act 1985 — which makes it 4.54609 ÷ 3.785411784 = 1.20095 times the US gallon, roughly 20 percent more fuel per gallon. UK MPG figures therefore need their own conversion constant: 4.54609 × 100 ÷ 1.609344 = 282.481.

A British review quoting 50 mpg means 282.481 ÷ 50 = 5.65 L/100km — the same car an American window sticker would rate at 50 ÷ 1.20095 = 41.6 MPG, with no change in how the engine actually drinks. Apply the US constant to the UK figure by mistake and you get 235.215 ÷ 50 = 4.70 L/100km, flattering the car by nearly a full liter per 100 km. Whenever an imported MPG figure looks suspiciously good, check its passport before its math.

Why one more MPG is worth more at 15 than at 50

MPG sits in the denominator of the thing you actually pay for. Fuel used equals distance ÷ MPG, so as MPG rises, consumption falls along a hyperbola — steep on the left, nearly flat on the right. Grant four different cars the same +5 MPG improvement and track the fuel each saves over 10,000 miles:

The identical badge-friendly "+5 MPG" saves 166.67 ÷ 18.18 ≈ 9.2 times more fuel at the bottom of the scale than at the top. Push the comparison further and it gets stranger: trading an 18 MPG truck for a 28 MPG one saves 555.56 − 357.14 ≈ 198 gallons per 10,000 miles, while trading a 34 MPG sedan for a 50 MPG hybrid — a leap that sounds far more heroic — saves 294.12 − 200.00 ≈ 94 gallons, less than half as much.

Consumers reliably misjudge this, and the failure has a name: the MPG illusion, documented by Larrick and Soll in Science (vol. 320, 2008). Their finding is the direct reason the EPA added gallons per 100 miles as a co-equal metric on the redesigned 2013 window sticker — America's homegrown cousin of L/100km, computed as 100 ÷ MPG and one of the five outputs the MPG calculator prints from a single fill-up.

The scale that moves like money

L/100km's real advantage is not geography — it is linearity. Your fuel bill is pump price times fuel consumed, and fuel consumed is proportional to the L/100km figure: double the consumption rating, double the bill. MPG offers no such straight line; the money hides behind the reciprocal.

Put prices on the table above. At the $3.50 per gallon the fuel cost calculator uses as its default pump price, the 15 → 20 upgrade returns 166.67 × $3.50 ≈ $583 per 10,000 miles, while the 50 → 55 upgrade returns 18.18 × $3.50 ≈ $64. Emissions track the same line, because CO2 is fixed by combustion chemistry at 8.887 kg per US gallon burned — the EPA's published tailpipe factor: 166.67 × 8.887 ≈ 1,481 kg of CO2 avoided at the low end versus 18.18 × 8.887 ≈ 162 kg at the high end. Same five MPG on paper; nine times the money and nine times the carbon in the world. That is why the fuel cost calculator accepts either MPG or metric L/100km through its units toggle — the question "what will this trip cost" only has a proportional answer in one of the two systems.

Choosing a lane

Neither unit is wrong; they answer different questions. Comparing against an American window sticker? MPG is the shared language. Budgeting a year of commuting, pricing a road trip, or estimating a CO2 footprint? Work in L/100km or gallons per 100 miles, where the arithmetic stays proportional to what you pay. Reading a British magazine? Convert the gallon before converting anything else. And when you have real data — miles driven and gallons pumped between two full tanks — enter it once and read all five scales side by side instead of chaining hand conversions; that habit prevents more unit errors than any formula. The constants on this page — 235.215, 282.481, 0.4251437 — are the same exact conversion factors the fuel-economy tools on Quanta compute with, so the arithmetic here and the number on screen trace back to one set of definitions.

The payoff for carrying one constant in your head is real. A rental brochure in Lisbon quoting 5.2 L/100km stops being foreign — 235.215 ÷ 5.2 ≈ 45.2 MPG, frugal in any market — and a pickup ad boasting three extra MPG prompts the only question that matters: three extra from where on the curve? For everything the constant can't settle, from diesel's different carbon chemistry to markets this page never visits, our contact page reaches the people who build these tools.

Sources

• U.S. EPA / DOE — official Fuel Economy Guide
• UK Weights and Measures Act 1985 — the imperial gallon fixed at 4.54609 litres
• U.S. EIA — weekly gasoline and diesel fuel update