The Gallon Problem: Why UK and US MPG Are 20% Apart

There are two gallons and nobody labels which one

A US gallon is 3.785 litres. An imperial gallon is 4.546 litres. The imperial one is about 20% larger, and both are simply called a gallon.

So the same car, burning the same fuel over the same distance, is rated 30 MPG in the United States and 36 MPG in the United Kingdom. Nothing improved crossing the Atlantic. The gallon got bigger.

This is where forum comparisons fall apart. Someone in Manchester reports 45 MPG, someone in Ohio reads it and wonders why their identical model manages 37. Both figures are correct. Neither is comparable.

The conversion is a single multiplication: imperial MPG equals US MPG times 1.201.

And a third system that measures the opposite thing

Canada, Australia and most of Europe use litres per 100 kilometres, which inverts the logic. MPG counts distance per fuel, so higher is better. L/100km counts fuel per distance, so lower is better.

Because the relationship is reciprocal rather than linear, the conversion is a division rather than a multiplication:

235.2 divided by US MPG gives L/100km. 282.5 divided by imperial MPG gives the same. Both constants run the other way too.

So 30 MPG in the US is 7.84 L/100km, and 8 L/100km is 29.4 MPG US or 35.3 MPG imperial.

The reciprocal relationship has a consequence people find genuinely counter-intuitive. Improving from 15 to 20 MPG saves more fuel over the same distance than improving from 40 to 50 MPG, despite being a smaller headline gain. Over 10,000 miles the first saves about 167 gallons and the second about 50. L/100km makes that obvious and MPG hides it, which is an argument for the metric system that the metric system rarely gets credit for.

The unit converter handles both directions if you would rather not remember which constant goes where.

Why your actual mileage is worse than the sticker

Official figures come from standardised test cycles, and standardised means repeatable rather than realistic. The US ratings on fueleconomy.gov are produced under controlled conditions, and the site is candid that real world results vary.

The gap is not random. It has causes you can name.

Cold starts are the big one for short journeys. An engine below operating temperature runs rich, and a two mile trip may never reach temperature at all. A commute made of short hops can run 20% below the rating with nothing wrong.

Speed matters more than most drivers expect, because aerodynamic drag rises with the square of velocity. The difference between 65 and 80 mph is far larger than the 23% speed increase suggests.

Then the mundane ones: underinflated tyres, a roof box left on all summer, air conditioning at full blast, a boot full of things you meant to take out in March.

Which is why the honest way to know your consumption is to measure it. Fill the tank, note the odometer, drive normally for a week, fill again, and divide. Two consecutive fills give a better number than any sticker.

What it actually costs you

Fuel economy only matters as money, and the arithmetic is worth doing once properly.

A car at 25 MPG driven 12,000 miles a year at $3.50 a gallon burns 480 gallons and costs $1,680. The same distance at 35 MPG burns 343 gallons and costs $1,200. A saving of $480 a year, and rather more if fuel goes up.

That number is what should sit next to the price difference when you are choosing between two cars, and it usually gets left out. So does the fact that fuel is rarely the largest cost of running a car. Depreciation almost always is, and it is invisible because nobody hands you a bill for it. Worth checking with the car depreciation calculator before optimising hard for a few miles per gallon.

For a specific trip rather than a year, the fuel cost calculator takes the distance, the economy figure and the price, and works in both MPG and L/100km, because a road trip that crosses a border crosses a unit system too.

When the fuel is somebody else's problem

If you drive for work, none of the above is quite the right calculation.

Employers generally reimburse per mile rather than per gallon, at a rate meant to cover fuel, wear, insurance and depreciation together. A more efficient car does not earn you more; it simply leaves more of the flat rate in your pocket.

The rate itself is set nationally rather than by your actual costs, and it moves. In 2026 the IRS split its standard business rate mid year, so a single average applied across the whole year quietly overpays one half and underpays the other. The mileage reimbursement calculator keeps the halves separate for that reason.

One thing worth knowing regardless of country: the trip you cannot claim is usually the commute. Home to your regular workplace is generally not business mileage anywhere in the English speaking world, however far it is.