Fuel economy is quoted three different ways in three different places, and only one of them scales the way intuition expects. The arithmetic is simple; the interesting part is why miles per gallon misleads people so reliably.
The conversion constant
A US gallon is 3.785411784 litres. A mile is 1.609344 kilometres. Put those together and litres per 100 km equals 235.215 divided by mpg.
The relationship is reciprocal, not linear, which is the source of all the confusion. Doubling mpg halves the fuel used; the two numbers move in opposite directions.
Imperial gallons are larger — 4.546 litres — so UK mpg figures are about 20 percent higher than US figures for the same car. Comparing a British road test to an American one without correcting for that is a common error.
Why mpg misleads
Consider two 10,000 mile years. Improving a vehicle from 15 to 20 mpg saves 167 gallons. Improving one from 30 to 50 mpg — a much more impressive-sounding change — saves 133 gallons.
The small improvement at the bottom of the scale saves more fuel than the large improvement at the top. That is entirely counterintuitive if you think in mpg, and obvious if you think in gallons per mile.
This has a real consequence: replacing the least efficient vehicle in a household saves more than upgrading the most efficient one, even for a smaller apparent gain.
Why we do not store fuel prices
Every calculator on this site takes prices as an input rather than carrying them. Fuel price is the clearest example of why.
A stored price would be wrong within days, and a page showing a stale number is worse than one showing none — it looks authoritative and is not. The same applies to material prices elsewhere on the site.
What does not change is the arithmetic and the conversion constants. Those are the same in ten years, which is the whole design principle here.
What actually moves real-world economy
Speed, above all. Aerodynamic drag goes with the square of speed and power to overcome it with the cube, so 80 mph uses far more fuel per mile than 65 — commonly 15 to 20 percent more.
Tire pressure, because underinflated tires increase rolling resistance measurably. Roof racks and open windows, for the same aerodynamic reason as speed. Cold weather, because engines run rich until warm and short trips never get there.
The rated figure on a window sticker is from a standardised test cycle. Real-world figures below it are normal and not a fault.
Cost per mile is the useful number
For comparing vehicles or deciding whether a trip is worth driving, cost per mile is more usable than mpg because it combines economy and price into one figure.
At 28 mpg and $3.50 a gallon that is 12.5 cents a mile in fuel. That is only the fuel — tires, maintenance and depreciation typically add two to four times as much again.
Which is why the fuel cost of a trip is usually the smaller part of what the trip actually costs, and a useful thing to know before optimising it.