Gallons per 100 miles (US) to Miles per gallon (UK)
gal/100mi (US)
mpg
Conversion History
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Quick Reference Table (Gallons per 100 miles (US) to Miles per gallon (UK))
| Gallons per 100 miles (US) (gal/100mi (US)) | Miles per gallon (UK) (mpg) |
|---|---|
| 1.5 | 0.09966545357418780779 |
| 2 | 0.13288727143225041039 |
| 2.5 | 0.16610908929031301299 |
| 3 | 0.19933090714837561559 |
| 4 | 0.26577454286450082079 |
| 5 | 0.33221817858062602598 |
| 6.7 | 0.44517235929803887482 |
About Gallons per 100 miles (US) (gal/100mi (US))
Gallons per 100 miles (US) — abbreviated gal/100mi — is a consumption metric, the American analogue to the European L/100km standard. Lower is better. It is the mathematical inverse of US mpg multiplied by 100. The US EPA introduced the GPM metric alongside mpg to help consumers make better fuel economy comparisons: because mpg is non-linear, the true fuel saving from improving a 15 mpg truck to 20 mpg (1.67 gal/100mi saved) is far larger than improving a 40 mpg car to 50 mpg (0.5 gal/100mi saved). GPM makes this intuitively clear by expressing absolute consumption rather than efficiency.
A 25 mpg family sedan consumes 4 gal/100mi. A 50 mpg hybrid uses only 2 gal/100mi — saving as much fuel per mile as the difference from 10 to 50 mpg on a low-efficiency vehicle.
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 (US) – Frequently Asked Questions
Why did the EPA start showing gallons per 100 miles on window stickers?
Research by Duke University professors showed that mpg systematically misleads consumers about fuel savings. The EPA added gal/100mi to Monroney stickers starting in 2013 to give buyers a linear, comparable number. A 1 gal/100mi difference always represents the same fuel saving regardless of starting point.
How does gal/100mi help compare plug-in hybrids that switch between fuel and electricity?
Plug-in hybrids complicate mpg because they drive some miles on electricity and some on petrol. Gal/100mi lets you weight the two modes transparently: if a PHEV uses 0 gallons for its first 40 electric miles and 2.5 gal/100mi after that, fleet managers can model exact fuel costs for any commute length. The EPA's MPGe tries to do something similar for consumers, but gal/100mi is more intuitive for mixed-mode budgeting.
What gal/100mi does the average American car use?
The average new car in the US uses about 3–4 gal/100mi (25–33 mpg). Full-size trucks sit around 5–7 gal/100mi, while hybrids achieve 1.5–2.5 gal/100mi. Electric vehicles show 0 gal/100mi but have a separate kWh/100mi rating.
Why do long-haul trucking companies obsess over fractions of a gal/100mi?
A Class 8 semi averages about 6.5 gal/100mi and covers 100,000+ miles per year — roughly 6,500 gallons annually. Shaving just 0.3 gal/100mi through aerodynamic fairings, low-rolling-resistance tires, or driver coaching saves ~300 gallons per truck per year. Across a 5,000-truck fleet, that is 1.5 million gallons saved — worth over $5 million at US diesel prices. At that scale, every tenth of a gal/100mi is a budget line item.
Why do Europeans never use gal/100mi even though they use L/100km?
Europeans adopted L/100km as their native metric from the start, so there was never a need for an alternative. Gal/100mi was invented specifically to fix the mpg illusion problem in the US. Europeans already had the "right" unit — they just used metric volumes and distances.
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.