Why “miles per gallon” is a backwards way to measure fuel
MPG hides how much fuel you actually save. The rest of the world uses liters per 100 km for a reason, and the difference changes which car upgrade is worth it.
5 min read · Reviewed August 2026
Miles per gallon feels like the obvious way to measure a car. Bigger number, better car. But MPG is quietly misleading, because it measures distance per fuel when the thing you actually pay for is fuel per distance. That flip fools almost everyone about where the real savings are.
The trap: MPG gains aren’t what they look like
Say you drive 12,000 miles a year. Improving a truck from 10 to 20 mpg cuts its fuel use from 1,200 gallons to 600, a saving of 600 gallons. Improving a car from 40 to 50 mpg cuts its use from 300 gallons to 240, a saving of 60 gallons. Both look like ‘plus 10 mpg,’ yet the first saves ten times the fuel. This is the MPG illusion, and it means fixing a gas-guzzler a little beats perfecting an efficient car a lot.
Why liters per 100 km is clearer
Most of the world measures fuel per fixed distance instead: liters to go 100 km. Lower is better, and the number tracks your actual spending in a straight line. Halve the figure and you halve the fuel bill. There’s no illusion to see through, because the unit is already pointed at the thing you care about. That’s the entire reason the rest of the world settled on it.
Converting between them
The catch is that the two aren’t a simple multiply, because one is the upside-down version of the other. To convert, divide 235.2 by the mpg figure to get liters per 100 km, and divide 235.2 by the liters figure to go back. That reciprocal is why our converter has a dedicated fuel-economy mode rather than a plain conversion factor. When you’re comparing two cars, compare them in fuel per distance, and the savings that MPG was hiding come right back into view.