How this instrument works
Miles per gallon grew out of British and American customary measure and became the fuel-economy figure quoted on US window stickers once the EPA began mandating economy labels on new cars in 1975, following the 1973 oil crisis. It answers a 'how far' question: more miles for the same gallon means a more efficient vehicle, so a bigger mpg number is always the better one.
Litres per 100 kilometres is the metric-world standard, used across Canada, Australia, the European Union and most of the rest of the globe. It answers the opposite question, 'how much fuel,' for a fixed distance, so a smaller L/100km figure is the better one. The two units aren't just different scales on the same ruler; they measure fuel economy along inverted axes, one distance-over-fuel and the other fuel-over-distance.
That inversion is why the conversion formula is a division rather than a multiplication by some tidy factor. The constant 235.214583 comes from multiplying 100 kilometres' worth of the unit system together: 100 × 3.785411784 litres per US gallon, divided by 1.609344 kilometres per mile. Divide that constant by an mpg figure and the reciprocal relationship falls out directly — L/100km = 235.214583 ÷ mpg.
- Enter your vehicle's rating into the Fuel economy (mpg, US) field. It's preloaded with 30, roughly a modern compact car's combined figure.
- Read the converted figure in the Fuel consumption (L/100km) field, recalculated the instant you change the mpg value.
- Remember the direction each unit favors: a higher number in Fuel economy (mpg, US) is better, while a lower number in Fuel consumption (L/100km) is better — they run opposite ways.
- Because the relationship is reciprocal, don't assume equal mpg gains save equal fuel. A jump from 15 to 30 mpg in the input field saves far more fuel than an identical 15 mpg jump from 45 to 60 would.
Worked example — a sedan, a hybrid, and a pickup truck
A compact sedan rated at 30 mpg is the default in the Fuel economy (mpg, US) field. Fuel consumption (L/100km) computes as 235.214583 ÷ 30 = 7.8404861, displayed as 7.84 — meaning that car burns about 7.84 litres to cover 100 kilometres.
Swap in a 50 mpg hybrid and Fuel consumption (L/100km) drops to 235.214583 ÷ 50 = 4.70429166, shown as 4.70. Going from 30 to 50 mpg, a 20 mpg gain, cut fuel use by 3.14 L/100km. A further 20 mpg gain, up to 70 mpg, would only shave off about 1.34 more L/100km, because the curve flattens as mpg climbs — the reciprocal relationship in action.
At the other end, a heavy pickup truck rated at just 15 mpg computes to 235.214583 ÷ 15 = 15.6809722, shown as 15.68 L/100km — over double the sedan's fuel use for the same distance, since halving the mpg figure exactly doubles the fuel-per-distance figure.
Questions
Why isn't converting mpg to L/100km a simple multiplication?
Because the two units measure fuel economy along opposite axes. Miles per gallon is distance divided by fuel, a 'how far' number where bigger is better. Litres per 100 km is fuel divided by distance, a 'how much' number where smaller is better. Flipping from one axis to the other is mathematically a reciprocal, y = k ÷ x, not a straight-line multiply, which is why the formula divides a constant by your mpg figure instead of multiplying it.
Where does the constant 235.214583 come from?
It's built from two exact metric-to-customary factors: 3.785411784 litres in one US gallon, and 1.609344 kilometres in one mile, both fixed by international agreement. Multiply 100 (for the '100 km' in the target unit) by the litres-per-gallon figure, then divide by the kilometres-per-mile figure, and the result is 235.214583, the number this calculator divides by your mpg reading.
Why does an equal mpg improvement save more fuel at low mpg than at high mpg?
Because the relationship is reciprocal rather than linear. Going from 10 to 20 mpg cuts fuel use from 23.52 to 11.76 L/100km, a savings of 11.76 L/100km. Going from 40 to 50 mpg, the same 10 mpg gain, only cuts fuel use from 5.88 to 4.70 L/100km, a savings of just 1.18 L/100km. The improvement is real in both cases, but the reciprocal curve means gains at the low end of the mpg scale matter far more to actual fuel consumed.
Does this use the US gallon or the UK imperial gallon?
The US gallon, at 3.785411784 litres exactly. The UK imperial gallon is larger, at roughly 4.546 litres, so a car rated in imperial mpg would need a different constant, about 282.481 instead of 235.214583, to convert correctly. Mixing the two produces numbers that look plausible but are wrong by about 20 percent.
Is a lower L/100km figure always better, unlike mpg?
Yes — L/100km inverts the usual 'bigger number, better economy' intuition that mpg trains into drivers. A car using 5 L/100km is more efficient than one using 8 L/100km, exactly the opposite direction from mpg, where the bigger figure wins. Anyone used to reading mpg stickers has to consciously flip that comparison when reading a metric fuel-consumption label.
Why did the EPA start mandating mpg labels in the first place?
The 1973 oil embargo exposed how little US consumers knew about vehicle fuel efficiency, and Congress responded with the Energy Policy and Conservation Act of 1975, which set the first Corporate Average Fuel Economy standards and required the window-sticker mpg ratings still printed on new cars today.