Gallons per 100 miles (UK) to Miles per gallon (US)
gal/100mi (UK)
mpg
Conversion History
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Quick Reference Table (Gallons per 100 miles (UK) to Miles per gallon (US))
| Gallons per 100 miles (UK) (gal/100mi (UK)) | Miles per gallon (US) (mpg) |
|---|---|
| 1.4 | 0.09302109001171117364 |
| 1.7 | 0.11295418072850642513 |
| 2 | 0.13288727144530167663 |
| 2.5 | 0.16610908930662709578 |
| 3 | 0.19933090716795251494 |
| 3.5 | 0.2325527250292779341 |
| 5 | 0.33221817861325419157 |
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 (US) (mpg)
Miles per gallon (US) — universally abbreviated mpg in the United States — is the dominant fuel economy metric in American automotive culture. Higher mpg means lower fuel consumption. The US gallon is 3.785 liters. The US Environmental Protection Agency (EPA) publishes city, highway, and combined mpg ratings on new vehicle window stickers, and Corporate Average Fuel Economy (CAFE) standards are set in mpg. Typical US passenger cars range from 15–20 mpg for trucks and large SUVs to 50–60 mpg for modern petrol hybrids. Because mpg is an efficiency unit (not consumption), the fuel savings from improving a low-mpg vehicle far exceed the savings from improving an already-efficient one.
A Ford F-150 pickup averages about 20 mpg combined on the EPA cycle. A Toyota Camry Hybrid achieves approximately 47 mpg combined.
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 (US) – Frequently Asked Questions
Why is EPA mpg always higher than what I actually get?
The EPA tests cars on a dynamometer in a lab, not on real roads. While EPA adjusted its formulas in 2008 to be more realistic, factors like cold weather, air conditioning, aggressive driving, hilly terrain, and short trips still cause most drivers to underperform the sticker by 10–20%.
Why does the US use miles per gallon instead of L/100km?
The US never adopted the metric system for everyday use, and mpg has been embedded in American car culture since the 1970s oil crisis when fuel economy became a selling point. CAFE standards codified mpg into federal law, making a switch politically and practically difficult.
What is the "mpg illusion" and why does it matter?
The mpg illusion is the cognitive bias where people assume equal mpg improvements save equal fuel. In reality, upgrading a truck from 12 to 14 mpg saves more gallons over 10,000 miles than upgrading a sedan from 30 to 50 mpg. This is because mpg is a reciprocal measure — savings are concentrated at the low end.
How do I convert US mpg to UK mpg?
Multiply US mpg by 1.201 to get UK mpg (because the imperial gallon is 20.1% larger than the US gallon). A car rated 30 US mpg is about 36 UK mpg. Many Americans visiting the UK are confused when British cars seem to get impossibly high mpg numbers.
What is the highest mpg car ever sold in the US?
The 2025 Hyundai Ioniq Blue held the record for non-plug-in cars at 59 mpg combined. Among hybrids, the Toyota Prius has consistently led, reaching 57 mpg combined in recent model years. Plug-in hybrids running on electricity achieve MPGe ratings over 100, but that is a different metric entirely.