Ask a British driver what a gallon is and they’ll usually pause. Britain’s been selling fuel by the litre since 1995, but the national instinct — what a tank of petrol “should” cost, how far you expect it to go — still lives in gallons and miles-per-gallon. Half the conversions you’ll find online don’t even get the gallon right.
The UK gallon is not the US gallon
Two definitions are in common use and they differ by almost twenty percent. An imperial (UK) gallon is 4.54609 litres. A US gallon is 3.78541 litres. When an American website quotes “$3.50 a gallon” and a British one quotes “155p a litre”, those numbers are closer than they look — but only after you convert both to the same gallon.
The short version: 1 UK gallon = 4.546 litres. Multiply the pump price per litre by 4.546 to get the UK gallon price. At today’s national average for E10 unleaded, a UK gallon runs roughly £7.10.
Conversion table
For a quick eyeball, at representative 2026 pump prices:
- 140p/L → £6.36 per UK gallon → £5.30 per US gallon
- 150p/L → £6.82 per UK gallon → £5.68 per US gallon
- 160p/L → £7.27 per UK gallon → £6.05 per US gallon
- 170p/L → £7.73 per UK gallon → £6.43 per US gallon
- 190p/L → £8.64 per UK gallon → £7.19 per US gallon (roughly where UK diesel sits right now)
MPG to L/100km, and back
European manufacturers quote fuel economy in litres per 100 kilometres; British drivers still think in miles per gallon. The conversion is non-linear — mpg is inversely proportional to L/100km — which is why you can’t just divide one by a constant.
The formula: L/100km × mpg (UK) = 282.48. Useful anchors:
- 5 L/100km ≈ 56 mpg (typical modern diesel hatchback)
- 7 L/100km ≈ 40 mpg (mid-size petrol saloon)
- 9 L/100km ≈ 31 mpg (large SUV)
- 12 L/100km ≈ 23 mpg (performance petrol)
If you only remember one number: 282. Divide it by your mpg to get L/100km; divide it by your L/100km to get mpg. The 0.48 rounding error is within measurement noise on any real trip.
Why Britain quotes pence per litre
The switch happened in stages. The Weights and Measures Act 1985 put the UK on a metric footing for trade. The 1994 Units of Measurement Regulations required fuel retailers to display prices in pence per litre. By October 1995 the gallon was no longer a legal unit for selling petrol, though it’s still permitted as a supplementary indication. That’s why you’ll occasionally see an older forecourt still displaying gallons alongside litres — it’s legal, just not common.
The practical bit: how far a tank goes
Tank sizes are quoted in litres on the spec sheet, but “range” is a gallon-brain calculation. A 55-litre tank (typical family hatchback) holds 12.1 UK gallons. At 45 mpg that’s 545 miles. At 35 mpg, 424 miles. The difference between those two figures is most of why fuel-efficient cars feel so much cheaper to run — not the pence-per-litre, but the gallons-per-trip.
Today’s UK national average, live →
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