Convert fuel economy instantly, with the working shown.
Every unit is converted to a common base unit first, then to your target unit: value × (from-factor) ÷ (to-factor).
MPG (miles per gallon) and km/L both measure distance travelled per unit of fuel — a "bigger number is better" unit. L/100km flips the relationship entirely, measuring fuel consumed for a fixed distance — a "smaller number is better" unit. Because MPG and km/L reward a larger number while L/100km rewards a smaller one, converting between them isn't a simple multiplication: the formula is inversely proportional (L/100km = 235.214583 ÷ MPG for imperial gallons), which is why going from 10 to 20 MPG cuts your fuel use in half, but going from 20 to 30 MPG — the same 10-unit increase — does not.
This inverse relationship has a genuinely practical consequence for comparing vehicles or judging an upgrade's value: L/100km-style units (and km/L, its inverse cousin) make fuel cost comparisons linear and intuitive in a way MPG never can. If Vehicle A uses 8 L/100km and Vehicle B uses 12 L/100km, you know instantly and exactly that B costs 50% more in fuel over the same distance — the numbers scale directly with cost. MPG doesn't offer that: the fuel savings from improving 10 to 20 MPG are dramatically larger than the savings from improving 30 to 40 MPG, even though both represent the same 10-unit MPG gain, which is a genuinely common source of buyers overestimating how much fuel money an MPG upgrade will actually save them at the higher end of the scale.
Because MPG rewards a larger number (more distance per fuel unit) while L/100km rewards a smaller number (less fuel per fixed distance) — they're inversely related, not directly proportional. The actual formula divides a constant by the MPG value (L/100km = 235.214583 ÷ MPG for imperial gallons) rather than using a simple multiplier.
Because of the inverse relationship between MPG and actual fuel consumed. A 10-unit MPG improvement represents a much bigger cut in litres-per-distance at the low end of the scale (10→20 MPG roughly halves fuel use) than the same 10-unit improvement at the high end (30→40 MPG), even though the MPG numbers increase by the same amount in both cases.
L/100km (or its cousin, litres-per-fixed-distance measures generally) is more directly useful for cost comparison, because fuel expense scales linearly with it — a car using 12 L/100km costs exactly 50% more in fuel than one using 8 L/100km over the same distance. MPG-style "distance per fuel unit" figures don't give you that same direct, linear cost relationship.
Yes — an advertised MPG improvement can sound more or less impressive than its actual real-world fuel savings, depending on where on the scale the improvement happens. A jump from a low starting MPG saves meaningfully more fuel than the same numeric jump from an already-high MPG, so it's worth converting to a consumption-based unit (L/100km or litres for your actual typical distance) before judging how much an upgrade will really save.