Fuel Economy Converter, Explained

After reading this you can convert any car's fuel figure between L/100 km, km/L, US MPG and UK MPG by hand, and you will know why the two scales behave so differently when you compare cars.

What fuel economy actually measures

There are two honest ways to describe how much fuel a car uses, and they are reciprocals of each other. One asks: how far do you go per unit of fuel? That gives you miles per gallon (MPG) and kilometres per litre (km/L). Bigger is better. The other asks: how much fuel does a fixed distance cost you? That gives litres per 100 km (L/100 km). Smaller is better.

The hook is that these two views disagree about which improvement matters. A car that goes from 20 to 25 US MPG saves more fuel over 10,000 miles than a car that goes from 40 to 50 US MPG, even though both look like a five-unit or ten-unit jump. The reason is the reciprocal, and once you see the arithmetic it stops being surprising.

When to use each unit

Use L/100 km when you want to budget fuel. It scales linearly with cost. If your commute doubles in distance, your litres double, and the L/100 km number tells you the litres directly. Most of Europe and Canada report this way.

Use MPG or km/L when you are comparing tank range or reading a US or UK spec sheet. The number is intuitive as "distance you can cover," but it hides savings, as the hook above shows.

The formulas and the intuition behind them

Everything rests on two constants: the distance in a mile and the volume in a gallon. One mile is 1.609344 km. A US gallon is 3.785412 L. A UK (imperial) gallon is 4.546090 L.

Start with the direct definition of US MPG. Miles per gallon means miles travelled divided by gallons burned. Convert a km/L figure into miles per US gallon:

Here km/L is your distance per litre, dividing by 1.609344 turns km into miles, and multiplying by 3.785412 turns "per litre" into "per US gallon." Combine the constants and you get a single number, 3.785412 / 1.609344 \approx 2.352.

Now link MPG to L/100 km. Litres per 100 km and km per litre are reciprocals scaled by 100:

Substitute that into the MPG formula. The 100 and the two constants collapse into the famous magic number:

Every symbol here is a pure unit conversion. The 235.215 is 100 \times 3.785412 / 1.609344. For UK gallons, replace 3.785412 with 4.546090 and the constant becomes 282.481.

The ratio of the two constants, 282.481 / 235.215 \approx 1.201, is exactly why UK MPG runs about 20% higher than US MPG for the same car. The car has not changed. The gallon has.

A worked example using the demo values

Why the MPG scale is curved

Because MPG is 235.215 divided by consumption, equal steps in MPG are not equal steps in fuel. Plot MPG against L/100 km and you get a hyperbola, not a straight line.

Work an example in fuel terms. Driving 10,000 miles (16,093 km) at 20 US MPG needs 10000/20 = 500 gallons. At 25 US MPG it needs 10000/25 = 400 gallons, a saving of 100 gallons. Now the "bigger" jump: 40 to 50 US MPG goes from 250 to 200 gallons, a saving of only 50 gallons. Same five-unit and ten-unit gaps, half the fuel saved. The curve is doing the work.

Reading and interpreting a converted result

Once you have the numbers, interpret them with the physics in mind, not just the arithmetic.

L/100 km

Litres burned per 100 km. Multiply by your annual distance in units of 100 km to get annual litres. At 8 L/100 km over 15,000 km that is 8 \times 150 = 1200 litres.

US MPG

Statute miles per 3.785 L. Realistic small cars sit near 30 to 40; large trucks near 15 to 20.

UK MPG

Statute miles per 4.546 L. Always about 1.20 times the US figure for the identical vehicle, so never compare a UK brochure to a US one without converting.

km/L

Kilometres per litre, common in Japan and India. Multiply by 2.352 to reach US MPG.

Common mistakes

The biggest error is treating US and UK MPG as the same unit. A car advertised at "42 MPG" in the UK is about 42 / 1.201 \approx 35 US MPG. Buyers who skip that step overestimate a UK-spec car's economy by a fifth.

The second mistake is averaging MPG figures directly. If you drive 100 km at 30 US MPG and 100 km at 10 US MPG, the correct combined figure is not (30+10)/2 = 20. Convert to fuel first. Distance 200 km is 124.3 miles. Gallons used: 62.14/30 + 62.14/10 = 2.071 + 6.214 = 8.285. Combined MPG is 124.3 / 8.285 = 15.0, not 20. The harmonic mean, not the arithmetic mean, is correct for MPG.

Where the idealized numbers break down

These conversions are exact unit algebra, but the published economy figures they convert are not. Test-cycle numbers (WLTP, EPA) assume a warm engine, mild acceleration and no headwind. Real driving in cold weather, city stop-and-go or at 120 km/h on a motorway can push actual consumption 15% to 40% above the sticker. The converter is faithful to the input; it cannot fix an optimistic input.

Electric vehicles report Wh/km or MPGe, and those are not fuel volumes at all, so this converter does not apply to them. For general energy and unit work reach for the Universal Unit Converter instead.

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