How this is worked out
Britain buys fuel by the litre, drives in miles and quotes economy in miles per imperial gallon. Three units, none of which cancel, which is why the calculation needs a conversion in the middle:
litres = distance in miles × 4.54609 ÷ MPG
cost = litres × price per litre
200 miles at 45 MPG is 20.21 litres — 4.44 imperial gallons — and at £1.42 a litre that is £28.69, about 14.3p a mile. Split four ways it is £7.17 each.
The imperial gallon is 4.54609 litres exactly. The US gallon is 3.785411784 litres exactly. They are 20.1% apart, so a UK MPG figure is always the flattering one:
45 MPG (imperial) = 37.5 MPG (US) = 6.28 L/100km
An American road test quoting 30 MPG is describing a car that a British magazine would call 36 MPG. Neither is wrong; they are different units wearing the same three letters.
A worked example
- Journey distance
- 200 miles, one way
- Fuel economy
- 45 MPG (imperial)
- Fuel price
- £1.42 a litre — a placeholder, not a quote
- Fuel used
- 20.21 litres — 4.44 imperial gallons
- Journey cost
- £28.69
- Cost per mile
- £0.14 — £14.35 per 100 miles
- Split four ways
- £7.17 each
- 8,000 miles a year
- £1,147.64 — £95.64 a month
- The same car at 60 MPG
- £860.73 a year — £286.91 saved
- 45 MPG (imperial) is also
- 37.5 MPG (US) · 6.28 L/100km · 15.93 km/L
Miles, litres and gallons in the same transaction
Britain half-completed metrication and stopped. Fuel has been sold by the litre since 1988, road signs are in miles, speedometers read miles per hour, and fuel economy is quoted in miles per imperial gallon — a unit no longer used to sell anything. It is the only country that routinely combines all three, and it makes every mental estimate a two-step conversion.
The consequence at the pump is that nobody knows what a fill costs per mile without doing arithmetic. A tank is priced in litres, the trip computer reports MPG, and the two do not meet without the 4.54609 factor. That is the conversion this page performs, and the reason the working is shown rather than just the answer.
The imperial gallon survives here for the same reason miles do: it was in the road signs and the adverts, and changing both at once was politically impossible. The 1985 Weights and Measures Act fixed it at exactly 4.54609 litres, which at least made the conversion exact.
Why a British MPG figure always looks better than an American one
The imperial gallon is 20.1% larger than the US gallon, so the same car travels 20.1% further on one. Every British MPG figure is therefore about a fifth higher than the American figure for the identical vehicle, and nothing in the units warns you.
This produces a specific, common error: reading an American review of a car sold here and concluding it is thirstier than it is, or reading a British review and expecting American figures. 30 MPG (US) is 36.0 MPG (imperial); 30 MPG (imperial) is 25.0 MPG (US). The four-unit table under the result exists to make that translation one glance rather than one calculation.
L/100km sidesteps the problem entirely, which is why every European manufacturer publishes it. It also happens to be the honest scale — see below.
What moves the price, and what does not
A large part of the pump price is tax and it is structured in two layers. Fuel duty is a fixed number of pence per litre — it does not rise or fall with the wholesale price — and VAT at the standard rate is then charged on the whole thing, duty included. So a penny of wholesale movement reaches the pump as slightly more than a penny, and a duty change reaches it multiplied by the VAT on top.
The rest is wholesale product cost and retailer margin, and the margin varies enormously by site. Supermarket forecourts, motorway services and rural independents can be 15p a litre apart on the same day, which on a 50-litre fill is £7.50 — comfortably more than most people save by driving carefully.
This is why the price field here is a placeholder rather than a researched figure, and why the page says so. Any number printed as "the current price of fuel" is wrong within a week, and the only price that matters is the one on the receipt in your pocket.
The MPG on the brochure, and the MPG on your dashboard
Official consumption figures come from the WLTP laboratory cycle, which replaced the older and far more optimistic NEDC test. WLTP is closer to reality than what it replaced, but it is still a standardised cycle at moderate speeds with no motorway cruising at 70 mph, no roof box and no headwind.
Real-world shortfalls of 10–20% are ordinary for petrol and diesel cars, and larger for plug-in hybrids, whose official figures assume the battery starts full. A plug-in hybrid driven without ever charging it produces figures nowhere near its label — it is carrying a heavy battery for nothing.
Your own figure is easy to measure and much more useful: brim the tank, note the mileage, brim it again next time, and divide the miles by the litres. Multiply litres by 0.219969 to get imperial gallons, or simply enter litres per 100 km on this page and read the MPG line.
Assumptions and sources
- Imperial gallon
- 4.54609 litres exactly, Weights and Measures Act 1985. https://www.legislation.gov.uk/ukpga/1985/72
- US gallon
- 3.785411784 litres exactly (231 cubic inches), for the conversion line.
- Mile
- 1.609344 kilometres exactly, international yard and pound agreement 1959.
- Fuel price
- The default here is a PLACEHOLDER, not a researched figure. Pump prices move weekly and vary by 15p a litre or more between retailers on the same day.
- Official consumption figures
- WLTP laboratory cycle. Real-world consumption is typically 10–20% worse, and much worse for an uncharged plug-in hybrid.
- Arithmetic
- tripCost, annualFuel and efficiencyIn in assets/js/tools/work.js, verified in tools/test/work.mjs.