Miles per Gallon (UK) (MPG (UK)) - Unit Information & Conversion

Quick Answer: What is Miles per Gallon (UK)?

Miles per gallon (UK) is the standard measurement for fuel economy in the UK. It tells you how far a car travels on one Imperial gallon of fuel.

• The "Imperial Gallon": It is 4.546 liters (larger than the 3.785-liter US gallon).
• The Difference: Because the gallon is bigger, UK MPG numbers are 20% higher than US numbers.
  • 50 MPG (UK) = 41.6 MPG (US)
• Metric Equivalent: 1 MPG (UK) ≈ 0.354 km/L.

Even though you buy petrol in liters in the UK, your car's dashboard almost certainly displays MPG.

MPG (UK) Comparison Table

Definition

Miles per Gallon (UK) is a unit of fuel economy expressing the distance in statute miles traveled per Imperial gallon of fuel.

Formula: MPG (UK) = Miles Driven ÷ Imperial Gallons Used

The Imperial Gallon: The Imperial gallon is exactly 4.54609 liters.

• It is about 1.2 times larger than the US liquid gallon.
• 1 Imperial Gallon ≈ 1.201 US Gallons.

Why this matters: If you read a US car review saying a car gets "30 MPG," that sounds low to a British driver. In UK units, that same efficiency would be "36 MPG."

History

1824: The Imperial Standard

Before 1824, there were different gallons for wine, ale, and corn. The British Weights and Measures Act of 1824 swept them all away and created one "Imperial Gallon" based on the volume of 10 pounds of water. This standard spread across the British Empire.

The Metric Mix

The UK began metricating in the 1960s and 70s.

• Roads: Distances stayed in miles.
• Fuel Pumps: Switched to liters in the 1980s.
• Cars: Continued to measure efficiency in MPG. This created a unique British hybrid system: you buy fuel in liters, drive in miles, and calculate economy in miles per gallon.

Modern Regulations

Today, official UK fuel economy figures (WLTP test cycle) are published in both L/100km (metric) and MPG (Imperial). However, virtually all marketing and consumer discussions focus on the MPG figure.

Real-World Examples

Typical UK Car Figures (2024)

• Petrol City Car: 45-55 MPG (UK) (e.g., Toyota Aygo)
• Diesel Family Car: 55-65 MPG (UK) (e.g., VW Golf TDI)
• Hybrid: 60-70+ MPG (UK) (e.g., Toyota Prius)
• Performance Car: 20-30 MPG (UK) (e.g., BMW M3)

The "40 MPG" Benchmark

For decades, 40 MPG was considered the benchmark for a decent, economical petrol car.

• Below 30 MPG: "Thirsty"
• Above 50 MPG: "Very economical"
• Above 60 MPG: "Excellent" (usually diesel or hybrid)

Canada's Usage

Canada officially uses L/100km, but many Canadians (especially older generations) still think in MPG. Crucially, they traditionally used the Imperial Gallon (like the UK), not the US gallon, so their mental benchmarks align with UK figures.

Common Uses

Car Dashboards

Almost every car sold in the UK, even from German or Japanese manufacturers, defaults to displaying MPG (UK) on the trip computer.

Advertising

"Up to 65 MPG!" is a standard headline in British car ads. Manufacturers know that L/100km (e.g., "4.3 L/100km") is not intuitive for most British buyers.

Used Car Market

When buying a second-hand car, the first question is often "What's the MPG?" It is the primary metric for affordability.

Conversion Mistakes to Avoid

1. The US/UK Mix-up

Mistake: Using a US online calculator for a UK car. Scenario: You calculate your fuel economy using a US website. It tells you "30 MPG." Reality: You actually got 36 MPG (UK). You are doing better than you thought!

• Always check if the calculator says "US Gallons" or "Imperial Gallons".

2. Converting L/100km Incorrectly

Mistake: Dividing 100 by the liters. Reality: The formula is 282.48 ÷ L/100km.

• Example: 5 L/100km.
• 282.48 ÷ 5 = 56.5 MPG (UK).

3. Confusing "Miles per Liter"

Mistake: Thinking MPG is miles per liter. Reality: Since fuel is sold in liters, some people try to calculate "miles per liter."

• 1 Mile per Liter ≈ 4.54 MPG (UK).
• If you get 10 miles per liter, that's roughly 45 MPG.

Unit Frequently Asked Questions

Why is petrol sold in liters but MPG uses gallons?

It's a quirk of British metrication. The UK adopted metric units for trade (selling fuel) but kept imperial units for road signs (miles) and tradition (gallons). It results in a confusing calculation:

1. Buy 40 liters of fuel.
2. Drive 400 miles.
3. Convert 40 liters to 8.8 gallons.
4. 400 ÷ 8.8 = 45.4 MPG.

What is a good MPG in the UK?

• Petrol: 45-50 MPG is good.
• Diesel: 55-60+ MPG is good.
• Plug-in Hybrid: Can show 100+ MPG, but this depends on how often you charge.

Is MPG (UK) used anywhere else?

Yes, informally in:

• Canada: Older drivers often use Imperial MPG.
• Caribbean: Some nations use Imperial units.
• Ireland: Officially metric (km/h and L/100km), but many drivers and cars still use MPG (UK).

How do I convert L/100km to MPG (UK)?

Divide 282.5 by the L/100km figure.

• 7 L/100km -> 282.5 / 7 = 40.3 MPG.
• 5 L/100km -> 282.5 / 5 = 56.5 MPG.

Why are diesel cars more popular in the UK for MPG?

Diesel fuel contains about 10-15% more energy per liter than petrol. This naturally gives diesel cars a higher MPG figure, which was very attractive to UK buyers when fuel prices were high.

Quick Reference Card

Your Next Steps

Master other fuel efficiency conversions:

• Miles per Gallon (US) - The American standard
• Liters per 100km - The European metric standard
• Kilometers per Liter - Another metric option
• Gallon (UK) - The volume unit itself