Why MPG and L/100km Can't Be Compared by Just Looking at the Numbers
Published 2026-10-04
Two units, two opposite directions
Miles per gallon measures how far you go per unit of fuel — a bigger number means better efficiency. Liters per 100 kilometers measures how much fuel you use per unit of distance — a smaller number means better efficiency. They're not just different scales of the same thing; they're inverted relative to each other, which makes comparing raw numbers across the two systems meaningless without converting first.
Why a trip's fuel cost still needs both distance and economy in matching units
Calculating how much fuel a trip uses requires dividing (or multiplying) distance by economy — and that only gives a sensible answer if both numbers are expressed in a compatible unit system. Mixing a distance in miles with an economy figure in L/100km without converting one of them first produces a nonsense result, even though the arithmetic itself runs without error.
Why fuel price isn't fetched automatically
Fuel prices change by the day, by region, and by station — any number hardcoded into a calculator would be wrong somewhere almost immediately. Entering your own current local price keeps the estimate grounded in reality rather than a stale average.
Plan your trip cost
Our Fuel Cost Calculator handles the MPG, L/100km and km/L conversions internally, so you can mix distance and economy units freely and still get a correct trip cost.