Making Sense of MPG Quoted in Imperial Gallons
A British review praises a hatchback for returning 52 mpg. An American review of what looks like the same car says 43. Neither writer is exaggerating and neither car has changed: the two figures are counted in gallons of different sizes, and the British one is measuring a bigger gallon.
The same gap shows up in a 1970s Haynes manual, on a classic car's original brochure, and in any Canadian document written before metrication. Once you know the gallon behind the number, the miles-per-gallon claim, the tank capacity and the service fluid quantities all fall into line with modern metric figures.
Why Britain Ended Up With a Bigger Gallon
A gallon defined by weight of water
Handbooks that outlived the unit
Economy that never went metric
Converting a Handbook Figure Into Metric Economy
Work from the document in front of you. Anything that quotes a volume goes straight through the two fields; anything that quotes economy needs the volume converted first, then divided by the distance.
Put the imperial gallon figure on the left
Enter the number as the manual prints it. Either a comma or a dot works as the decimal separator, and any spaces you type are discarded before the value is read.
Take the litre value as it appears
There is no button to press — the second box tracks your typing. Results carry up to eight decimals, which matters when you are checking a sump capacity against a modern service schedule.
Reverse it for a modern spec sheet
Type into the right-hand box to read litres back as imperial gallons, or press the swap button (↔) to flip the pair for good when the newer document is the one you are working from.
Lift the number into your restoration notes
The copy button on each field gives you the digits by themselves, no unit and no thousands spacing, ready to drop into the rebuild log or the spreadsheet of fill quantities. Ctrl + C works the same way.
To turn a claimed economy figure into metric by hand, divide 282.481 by the imperial mpg. The constant is just 100 × 4.54609 ÷ 1.609344, so it already contains both the gallon and the mile.
Imperial MPG, L/100 km and the US Figure Side by Side
Read across a row to see the same car described three ways. The imperial column is what a British magazine or an on-board computer reports; the middle is what a European spec sheet shows; the right-hand column is the figure an American window sticker would carry.
| MPG (imperial) | L/100 km | MPG (US) | The sort of car that returns it |
|---|---|---|---|
| 20 | 14.12 | 16.7 | Large petrol V8, or a classic driven hard |
| 25 | 11.30 | 20.8 | Big petrol SUV or an older estate |
| 30 | 9.42 | 25.0 | Mid-size petrol saloon in town |
| 35 | 8.07 | 29.1 | Small petrol hatchback, mixed driving |
| 40 | 7.06 | 33.3 | Modern petrol hatch on a steady run |
| 45 | 6.28 | 37.5 | Efficient petrol or a small diesel |
| 50 | 5.65 | 41.6 | Family diesel on a motorway cruise |
| 60 | 4.71 | 50.0 | Hybrid, or a small diesel driven gently |
The two mpg columns keep a constant ratio of 1.20095, so a quick sanity check is to knock a sixth off any British claim before comparing it with an American one. The metric column will not scale that way, because it counts fuel per distance rather than distance per fuel.
What Helps When You Compare Economy Claims
Instant answers while you read
The second field follows every keystroke, so working down a column of capacities in an old handbook does not mean pressing anything between entries.
Old document or new, same two boxes
Both fields accept typing and the ↔ button reverses the pair, so a modern litre spec and a period gallon spec can be checked against each other in seconds.
US gallons and pints in the same lists
The searchable dropdowns carry all 28 units, so an imperial figure can be pushed straight to US gallons when you are comparing a British and an American brochure.
Numbers ready for a build sheet
Each copy button yields the plain value without a unit, which is what a restoration log or a fluid-capacity table in a spreadsheet expects.
Questions About British Fuel Figures
Why does a British mpg number flatter the same car?
Because the gallon in it is 20.095% bigger. A car covering a set distance on one imperial gallon covers the same distance on 1.20095 US gallons, so the British figure is inflated by that factor before anything else is considered. A 48 mpg claim in London is 40 mpg in Los Angeles. Divide any British number by 1.20095 to make it comparable, and be sceptical of side-by-side tables that do not say which gallon they mean.
Where did the imperial gallon's odd size come from?
From water. The Weights and Measures Act of 1824 swept away a tangle of ale, wine and corn gallons and defined a single one as the volume occupied by ten pounds of distilled water under stated conditions, which worked out at about 4.546 L. That is the origin of the old saying that a pint of water weighs a pound and a quarter. The definition was later tidied up to the exact figure of 4.54609 L used today.
Is the imperial gallon still a legal measure in the UK or Canada?
Not as the primary unit for selling fuel. Britain moved forecourts to litres at the end of the 1980s and Canada did so around 1979, and both now require metric quantities on the pump. The gallon survives in Britain as a supplementary indication and in everyday speech, which is why economy is still advertised in miles per gallon while every litre is metered metrically. Canada dropped it far more thoroughly and quotes economy in L/100 km.
How do I read the fluid capacities in an old British manual?
Assume every volume in it is imperial unless the book says otherwise, and watch for pints, which appear far more often than gallons in service sections. An imperial pint is 568.26 mL, an eighth of the gallon, so a sump quoted as "7 pints" is 3.98 L rather than the 3.31 L an American reader would calculate. Fuel tanks, cooling systems and gearbox fills all follow the same rule, and getting the pint wrong is the classic mistake on a first rebuild.
Why do L/100 km and mpg move in opposite directions?
They are reciprocals of one another. Miles per gallon asks how far a fixed amount of fuel takes you, so bigger is better; litres per 100 km asks how much fuel a fixed distance costs, so smaller is better. That also means the scales stretch differently. Going from 20 to 25 mpg saves about 2.8 L per 100 km, while the same five-point jump from 50 to 55 mpg saves only about 0.5 L — which is why fuel-saving effort pays off most on the thirstiest vehicles.
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