UK vs US MPG: Why a US Gallon Is Not a UK Gallon

A UK (imperial) gallon is 4.54609 litres and a US gallon is 3.785411784 litres, so the UK gallon is about 20% bigger. The same car therefore shows a lower mpg in US figures. Multiply UK mpg by 0.832674 to get US mpg: 36.0 UK mpg is 30.0 US mpg.

What is the difference between a UK gallon and a US gallon?

A UK gallon holds 4.54609 litres. A US gallon holds 3.785411784 litres. Divide one by the other and the UK gallon is 1.2009 times the size of the US gallon.

Miles are the same in both countries, at 1.609344 km. Only the gallon differs. A car that uses a certain amount of fuel per mile will therefore cover more miles on a UK gallon than on a US one, simply because the UK gallon holds more fuel.

Why are US mpg figures lower for the same car?

Because the US gallon is smaller, the same fuel economy gives a smaller number of “miles per gallon”. It is a unit difference, not a performance difference. Nothing has changed about the car.

The conversion is UK mpg x 0.832674 = US mpg, or US mpg x 1.200950 = UK mpg. Both use the exact litre values above. A vehicle doing 50 UK mpg uses 5.65 litres per 100km. In US figures that is 41.6 mpg.

What is the UK to US mpg conversion table?

Use these tables to move between the two. Figures are rounded to one decimal place.

UK mpg	US mpg
30	25.0
35	29.1
40	33.3
45	37.5
50	41.6
55	45.8
60	50.0

US mpg	UK mpg
20	24.0
25	30.0
30	36.0
35	42.0
40	48.0
45	54.0
50	60.1

A quick check from US mpg to UK mpg: add a fifth. 30 US mpg plus a fifth is 36 UK mpg. The exact factor is 1.2009, so the shortcut is close enough for a quick read.

What are the traps when reading US car reviews and EPA figures?

The biggest trap is quoting a US figure next to a UK one as if they were the same unit. A US review that praises 35 mpg is describing 42.0 UK mpg. A UK driver who compares that to a 38 mpg UK figure will think the American car is worse when it is better.

Other traps to watch for:

• Check which gallon. If a source does not say, assume a US publication uses the US gallon and a UK one uses the imperial gallon.
• Different test cycles. US figures from the Environmental Protection Agency (EPA) and UK figures from the WLTP test come from different procedures. Even after converting the gallon, the two numbers are not directly comparable. The gallon fix removes one difference, not all of them.
• Electric and hybrid labels. Some US labels use an energy equivalent figure for electric vehicles. That is a separate measure and the gallon table above does not apply to it.
• Fuel price per gallon. US prices are quoted per US gallon. At 174.07p a litre, DESNZ petrol for the week of 26 October 2026, a US gallon of fuel would cost £6.59 and a UK gallon £7.91. If you compare prices, make sure the volume matches.
• Rounded figures. Some conversions online use 0.83 or 1.2 as shortcuts. Over a large fleet these small errors add up, so use the exact factors.

How do you convert the gallon yourself?

Convert everything to litres, then back to the unit you want. Take 30 US mpg. In litres per 100km that is 235.214583 / 30 = 7.84. In UK mpg, 282.480936 / 7.84 = 36.0.

So 30 US mpg is 7.84 L/100km and 36.0 UK mpg. L/100km is the cleanest middle step because it is the same everywhere. Our guide to mpg to litres per 100km has the full formula and table.

Which should a UK fleet use?

Use UK mpg, litres per 100km or pence per mile, but never mix them in one report. A UK pump sells fuel by the litre in pounds, and the cost sum works from litres. Cost per mile at a UK pump price is price per litre in pence x 4.54609 / UK mpg. If you start from a US figure, convert it first.

The fuel cost calculator converts mpg, L/100km, km per litre and US mpg, and it is pre-filled with this week’s DESNZ prices. To measure your own vehicles properly, see how to calculate mpg. To see why published figures do not match your results, read why real-world mpg is lower than official.

Frequently Asked Questions

Is a US gallon the same as a UK gallon?

No. A US gallon is 3.785411784 litres and a UK (imperial) gallon is 4.54609 litres. The UK gallon is about 20% larger. Miles are the same in both systems.

How do I convert US mpg to UK mpg?

Multiply US mpg by 1.200950. So 30 US mpg is 36.0 UK mpg. To go the other way, multiply UK mpg by 0.832674.

Why does the same car have a lower mpg in the US?

Because the US gallon holds less fuel, so each gallon gives fewer miles. The car is the same. Convert the figure to litres per 100km to see that the fuel used per distance has not changed.

Are EPA mpg figures the same as UK official mpg?

No. They are measured under different test procedures as well as in different gallons. Converting the gallon removes one difference but not the other, so treat the converted number as a rough guide, not an exact match.

Which mpg do UK car adverts and fleets use?

UK adverts and official figures use imperial mpg. Fleets can also use litres per 100km, which many fleet systems report. Pick one unit and use it everywhere in your reports.

Cut your fleet’s fuel bill

Find your own number with the fuel cost calculator. Enter your vehicle’s mpg in whichever unit you have, and it shows the cost per mile and the annual bill using this week’s DESNZ prices.

A speed cap saves fuel on every journey without the driver having to remember it. The Department for Transport’s fuel consumption curves (TAG data book v2.03, May 2026) show a diesel van using 14.5% less fuel per mile at 60 mph than at 68 mph. How much of that reaches your bill depends on how many of your miles are driven above the cap. See fuel savings from speed limiters.

Talk to us. AutoKontrol has made speed limiters in Britain since 1985, with nationwide installation where we come to you. Book a free consultation or get a quote.