Lower is better, or higher?
Fuel economy is measured two opposite ways round, and mixing them up inverts the comparison while still producing a plausible number.
Miles per gallon and kilometres per litre measure distance per volume — how far you get on a fixed amount of fuel. Bigger is better.
Litres per 100 kilometres measures volume per distance — how much fuel a fixed journey costs. Smaller is better. It is the reciprocal of the first pair rather than a rescaling of it, so no multiplying factor connects them and the two scales run in opposite directions.
The practical consequence: a car at 5 L/100 km is better than one at 8, while a car at 50 mpg is better than one at 30. Someone reading an unfamiliar figure has an even chance of reading the comparison backwards, which is why this tool labels the direction on every unit rather than leaving it to be inferred.
A useful anchor for converting in your head: 235 divided by US mpg gives L/100 km, and dividing back gets you home again, because a reciprocal is its own inverse. 30 mpg is about 7.8, and 235 ÷ 7.8 is back to 30. For UK mpg the constant is about 282, because the gallon is larger.
A UK 40 mpg is a US 33 mpg
Both countries write “mpg”, and they do not mean the same thing. An imperial gallon is 4.546 litres and a US gallon is 3.785 — about 20% apart — so the same car covers more miles per imperial gallon simply because the gallon holds more fuel.
A British review quoting 40 mpg is describing a car an American review would quote at 33.3 mpg. Nothing about the car differs, nothing in the text marks the change, and the figure crosses the Atlantic in a spec sheet or a forum post with no unit to correct it.
The direction is worth memorising, because it flatters. A UK figure always reads higher than the equivalent US one, so a British car quoted at 60 mpg is a US 50 — impressive either way, but not the same claim. This converter lists the two as separate units for exactly the reason the volume converter refuses to offer an unnamed “gallon”.
Canada sidesteps the problem entirely by using L/100 km despite bordering the largest mpg market, and Australia and most of Europe do the same.
Why small mpg gains beat big ones
This is the one that surprises people, and it is a large part of why regulators moved to L/100 km. mpg is not linear in fuel consumed, so two upgrades that look equally large on the label are not equally valuable at the pump.
Over 15,000 miles a year:
- Going from 10 to 15 mpg saves about 1,893 litres.
- Going from 30 to 50 mpg saves about 757 litres.
A five-mpg improvement at the bottom of the range saves two and a half times as much fuel as a twenty-mpg improvement at the top. The reason is that fuel used is proportional to one over mpg, so the same numerical step is worth far more where the denominator is small.
The practical version: replacing an old van at 10 mpg with one at 15 does more for a fleet’s fuel bill than replacing an efficient car at 30 with a hybrid at 50. That is genuinely counterintuitive from the labels, and it stays invisible until you convert to fuel consumed.
L/100 km carries no such illusion, because it is fuel consumed: a saving of 1 L/100 km is the same amount of fuel wherever it happens on the scale. The table in the tool computes that comparison rather than asserting it, so the numbers on screen are the argument.
Label figures and real driving
A converted number is only as good as the number it started from, and official economy figures are test results rather than measurements of driving.
European and UK figures changed test regime between 2017 and 2019, from NEDC to WLTP. WLTP is longer, faster and more demanding, so the same car typically reads 10–20% worse under it. A 2016 brochure and a 2020 brochure for the same model are therefore not comparable, and the car did not become thirstier in between.
US EPA figures come from their own cycles with adjustment factors applied before publication, and are generally closer to real driving than pre-WLTP European figures were — but they remain a standardised test, and the agency says so plainly.
Real consumption then moves with things no test captures: speed, since drag rises with its square; load; terrain; tyre pressure; air conditioning; and cold starts. A short winter commute can be dramatically worse than any label, because the engine never reaches operating temperature at all.
So use a converted label figure to compare cars tested the same way, and your own brim-to-brim measurement to know what your car actually does.
Doing it in a spreadsheet
The two constants worth keeping: =235.215/A1converts US mpg to L/100 km and back again, and =282.481/A1 does the same for UK mpg. Both are reciprocal formulas, so the identical expression works in either direction — a neat property, and also the thing that makes the units confusing.
Between the two mpg figures it is a straight factor: =A1*0.832674 turns UK mpg into US mpg and =A1*1.200950 goes back, both exact ratios of the two gallons.
For a running-cost sheet, work in fuel used rather than in economy: =distance/economy for mpg, or =distance/100*L100km for the metric form. Averaging a column of mpg figures across journeys gives the wrong answer for the same reason averaging speeds does — divide total fuel by total distance at the end instead.
Sources and methodology
Nothing here is fetched and there is no data feed. The conversion factors are exact — the 1959 mile and both statutory gallons — so the arithmetic carries no error. What the sources settle is the part that does: that label figures come from defined test cycles rather than from driving, and that the cycle changed in Europe between 2017 and 2019, so two figures for the same car may not be comparable.