Gallons per 100 miles (UK) to Miles per gallon (UK)
gal/100mi (UK)
mpg
Conversion History
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Quick Reference Table (Gallons per 100 miles (UK) to Miles per gallon (UK))
| Gallons per 100 miles (UK) (gal/100mi (UK)) | Miles per gallon (UK) (mpg) |
|---|---|
| 1.4 | 0.11171367112003154953 |
| 1.7 | 0.13565231493146688157 |
| 2 | 0.15959095874290221362 |
| 2.5 | 0.19948869842862776702 |
| 3 | 0.23938643811435332043 |
| 3.5 | 0.27928417780007887382 |
| 5 | 0.39897739685725553404 |
About Gallons per 100 miles (UK) (gal/100mi (UK))
Gallons per 100 miles (imperial) is the British consumption-oriented counterpart to the metric L/100km standard, expressing how many imperial gallons a vehicle burns over 100 miles. It is numerically distinct from its US equivalent because the imperial gallon (4.546 L) is larger than the US gallon (3.785 L), so UK gal/100mi values are roughly 20% lower than US gal/100mi for identical fuel consumption. The unit is used in UK engineering and fleet management contexts where consumption rather than efficiency framing is preferred, and in academic analysis of British vehicle fuel-use data.
A 50 UK mpg car consumes 2 gal/100mi (imperial). A typical British family SUV at 35 UK mpg uses about 2.86 gal/100mi (imperial).
About Miles per gallon (UK) (mpg)
Miles per gallon (imperial) — also written UK mpg — is the traditional British fuel economy unit, using the imperial gallon of 4.546 liters rather than the smaller US gallon (3.785 L). Because the imperial gallon is approximately 20% larger, UK mpg figures are roughly 20% higher than US mpg for the same vehicle: a car rated at 40 US mpg is approximately 48 UK mpg. The UK officially adopted L/100km for type-approval testing under EU harmonisation, but UK mpg remains prevalent in British automotive journalism, used-car listings, and consumer conversations in the United Kingdom and Ireland.
A typical family hatchback in the UK achieves 45–55 mpg (imperial) on the WLTP combined cycle. A 60 UK mpg rating equates to approximately 50 US mpg or 4.7 L/100km.
Gallons per 100 miles (UK) – Frequently Asked Questions
Why would anyone use imperial gallons per 100 miles?
Fleet managers and logistics companies in the UK sometimes prefer this consumption-based metric because it scales linearly with cost. If fuel costs £5.50 per imperial gallon, a vehicle using 3 gal/100mi costs exactly £16.50 per 100 miles — much easier to budget than working backwards from mpg.
How does UK gal/100mi compare to US gal/100mi for the same car?
UK gal/100mi values are about 17% lower than US gal/100mi because the imperial gallon is larger. A car consuming 4 US gal/100mi uses only about 3.33 UK gal/100mi. Same fuel burned, but fewer of the bigger gallons needed to measure it.
How do UK fleet operators use gal/100mi to set carbon budgets per vehicle?
Burning one imperial gallon of diesel produces about 10.2 kg of CO₂. Multiplying gal/100mi by that emission factor gives a direct kg-CO₂-per-100-miles figure for each vehicle. Fleet managers set thresholds — say 25 kg CO₂/100mi — and flag any vehicle exceeding it for maintenance or driver retraining. The linear relationship makes gal/100mi far better than mpg for carbon accounting, where small absolute savings matter.
What is a good UK gal/100mi for a family car?
A modern family hatchback should achieve around 1.8–2.3 UK gal/100mi (43–56 UK mpg). Anything under 2 gal/100mi is considered very efficient. SUVs typically sit at 2.5–3.5 gal/100mi, and performance cars can exceed 4 gal/100mi.
Is this unit used on any official UK documentation?
Not on consumer-facing documents. UK vehicle registration (V5C) and type-approval certificates show L/100km and CO₂ g/km. Gal/100mi (imperial) lives mainly in fleet management software, academic transport research, and engineering reports where linear consumption comparisons matter more than the traditional mpg framing.
Miles per gallon (UK) – Frequently Asked Questions
Why is UK mpg different from US mpg for the same car?
The imperial gallon (4.546 L) is 20.1% larger than the US gallon (3.785 L). So a car that travels 40 miles on a US gallon would travel 40 miles on less than one imperial gallon, yielding a higher UK mpg number. Same car, same fuel, different gallon definitions — endlessly confusing.
Is UK mpg still used now that Britain uses liters at the pump?
Yes, stubbornly so. British fuel is sold in liters and road signs show miles, but most Brits still discuss fuel economy in mpg (imperial). It is a quirky holdover — they buy liters, drive miles, and mentally calculate in a unit that directly matches neither.
How do I convert UK mpg to L/100km?
Divide 282.5 by the UK mpg value. So 50 UK mpg equals 282.5 ÷ 50 = 5.65 L/100km. For US mpg, divide 235.2 instead. These magic numbers are worth memorising if you cross-shop cars between markets.
Why did UK diesel car sales collapse after 2017 despite their higher mpg?
The Volkswagen emissions scandal (2015) shattered public trust in diesel. The UK government then announced clean-air zones charging diesel cars extra, raised vehicle excise duty on new diesels, and signalled a 2030 petrol/diesel ban. Residual values cratered, making diesel a bad financial bet despite 15–20% better mpg. Diesel's UK market share fell from 48% in 2015 to under 10% by 2023, replaced by hybrids and EVs.
Why do British car magazines show different mpg than the manufacturer claims?
Manufacturers quote official WLTP lab test results, while magazines run real-world tests with varying conditions. Magazine figures are usually 10–20% worse than WLTP because they include cold starts, traffic, motorway speeds above test assumptions, and actual British weather.