Two units measuring the same thing, opposite ways round
Miles per gallon counts how far a car goes per unit of fuel: bigger is better. Litres per 100 kilometres counts how much fuel a car burns per unit of distance: smaller is better. Both are legitimate ways to describe the same physical property of a car, and they are reciprocals of one another, not two points on a shared linear scale. That single fact is responsible for a genuine, quantifiable illusion in how fuel-economy improvements get reported and understood.
The exact conversion, and why the constant differs by gallon
Converting requires a gallon and a mile. The international mile has been exactly 1.609344 km since the 1959 international yard-and-pound agreement, so:
L/100km = (100 × gallon size in litres) ÷ (1.609344 × mpg)
Using the US gallon (3.785411784 L, see the gallon-problem guide on this site): 100 × 3.785411784 ÷ 1.609344 = 235.2146 ÷ mpg(US). Using the Imperial gallon (4.54609 L exactly): 100 × 4.54609 ÷ 1.609344 = 282.481 ÷ mpg(Imp). Two different constants for what looks like the same word "mpg," because the two gallons are two different volumes.
The MPG illusion, worked in full
Take a fixed distance, 10,000 miles, and compute the fuel a car uses at several mpg ratings: fuel used = distance ÷ mpg.
| Fuel economy | Gallons used over 10,000 mi | Gallons saved vs. previous row |
|---|---|---|
| 10 mpg | 1,000.00 | — |
| 20 mpg | 500.00 | 500.00 |
| 30 mpg | 333.33 | 166.67 |
| 40 mpg | 250.00 | 83.33 |
| 50 mpg | 200.00 | 50.00 |
| 100 mpg | 100.00 | 100.00 (50→100) |
Read the two halves of the brief's own comparison directly off this table. Improving from 10 to 20 mpg saves 1,000.00 − 500.00 = 500 gallons over 10,000 miles. Improving from 30 to 50 mpg — a bigger jump in mpg terms, +20 rather than +10 — saves only 333.33 − 200.00 = 133.33 gallons over the same distance, less than a third as much fuel, despite being the "larger" improvement by the number printed on the window sticker.
Why: mpg is a reciprocal, and reciprocals compress at the high end
Fuel used for a fixed distance D is D ÷ mpg, and the rate of change of that quantity with respect to mpg is −D ÷ mpg²: the gallons saved per additional mpg shrinks with the square of the mpg you're improving from. At 10 mpg, each additional mpg is worth a lot of saved fuel; at 50 mpg, each additional mpg is worth very little, because the car was already close to using almost no fuel per mile. A linear-looking scale (10, 20, 30… mpg) hides a curve that is steep at low values and almost flat at high ones. L/100km does not have this problem, because it is directly proportional to fuel consumed per fixed distance: a drop from 10 to 8 L/100km and a drop from 5 to 3 L/100km represent exactly the same 2 L/100km of real fuel saved per 100 km driven, which is why most of the world outside the US regulates and labels fuel economy in L/100km (or the equivalent km/L) rather than a distance-per-fuel figure.
US mpg and Imperial mpg: the same car, two different numbers
Because the two gallons differ in size, "mpg" printed on a US spec sheet and "mpg" printed on an older UK one are not the same measurement even for an identical car with identical real-world efficiency. The ratio between them is fixed by the ratio of the gallons themselves: 4.54609 ÷ 3.785411784 = 1.20095, so Imperial mpg reads about 20.1% higher than US mpg for the same physical fuel efficiency. Worked through a real figure, carried without rounding the intermediate step — the discipline the significant-figures guide argues for: a car rated at 40 mpg(US) converts to 235.2146 ÷ 40 = 5.880365 L/100km, which converts back to Imperial mpg as 282.481 ÷ 5.880365 = 48.038 mpg(Imp). The same car, unchanged, is "40 mpg" in the US and "48 mpg" under the Imperial convention — not because it burns less fuel, but because the gallon being counted is 20% bigger, exactly the relationship worked out with fluid ounces and gallons in the gallon-problem guide.
The view this site takes
L/100km is the more honest of the two units, not merely a different convention. It is linear in the thing that actually costs money and produces emissions — fuel burned per distance driven — while mpg is linear in the opposite quantity, distance per fuel burned, which flatters improvements at the top of the range and understates them at the bottom, exactly where replacing an inefficient vehicle matters most for total fuel consumption. A regulator or a buyer comparing a 10 mpg truck against a 20 mpg truck is looking at a far bigger real saving than a 30 mpg sedan buyer trading up to 50 mpg, and only L/100km shows that saving as the bigger number it actually is.