Fuel Economy Units Explained
Fuel economy is quoted in at least three incompatible ways, two of them run in opposite directions, and the word "gallon" means two different volumes depending on which side of the Atlantic you are on.
In short
- L/100km measures consumption: lower is better.
- MPG and km/L measure efficiency: higher is better.
- A US gallon is 3.785 L; a UK gallon is 4.546 L — about 20% larger.
- Equal MPG gains do not save equal fuel; the improvements at the bottom of the scale are worth far more.
Two ways round, and they are not interchangeable
There are only two things you can measure: how much fuel it takes to cover a distance, or how much distance you get from a unit of fuel. Litres per 100 km is the first. Miles per gallon and kilometres per litre are the second.
Because they are reciprocals, they move in opposite directions. A car improving from 8 to 6 L/100km is getting better; a car improving from 30 to 40 MPG is also getting better. Reading a table without checking which convention it uses is an easy way to draw exactly the wrong conclusion.
The two gallons
The US gallon is 3.785411784 litres. The imperial gallon still used in the UK is 4.54609 litres — about 20% larger. The same car is therefore quoted at noticeably different MPG figures in the two countries, without anything about the car changing.
| UK MPG | US MPG | L/100km | km/L |
|---|---|---|---|
| 30 | 25.0 | 9.4 | 10.6 |
| 40 | 33.3 | 7.1 | 14.2 |
| 50 | 41.6 | 5.6 | 17.7 |
| 60 | 50.0 | 4.7 | 21.2 |
| 70 | 58.3 | 4.0 | 24.8 |
A car advertised at 40 MPG in Britain would be advertised at about 33 MPG in the United States. Neither figure is wrong, and comparing them directly is the mistake.
Converting between them
The conversions are single constants, which is worth knowing because it means you can sanity-check any figure quickly:
L/100km = 235.21 ÷ US MPG · L/100km = 282.48 ÷ UK MPG
So 40 US MPG is 235.21 ÷ 40 = 5.9 L/100km, and 40 UK MPG is 282.48 ÷ 40 = 7.1 L/100km. The same constants work in reverse: divide the constant by the L/100km figure to get MPG back.
Kilometres per litre is simpler: km/L = 100 ÷ L/100km, and a US MPG figure divided by 2.352 gives km/L directly.
Why MPG misleads about savings
This is the part that genuinely surprises people. Because MPG is a reciprocal, equal improvements in MPG do not produce equal fuel savings. The gains at the low end of the scale are worth far more.
Take a fixed 20,000 km a year and compare two upgrades that each add 10 MPG:
| Upgrade | L/100km before | L/100km after | Litres saved per 20,000 km |
|---|---|---|---|
| 15 → 25 US MPG | 15.7 | 9.4 | 1,254 |
| 25 → 35 US MPG | 9.4 | 6.7 | 538 |
| 35 → 45 US MPG | 6.7 | 5.2 | 299 |
| 45 → 55 US MPG | 5.2 | 4.3 | 190 |
The first upgrade saves more than six times as much fuel as the last, despite both being "10 more MPG". Replacing a genuinely thirsty vehicle with a merely average one saves far more than turning an efficient car into a very efficient one — which is the opposite of where attention usually goes.
This is sometimes called the MPG illusion, and it is the reason several countries moved their official labelling to a consumption-based figure like L/100km or gallons per 100 miles, where equal changes really do mean equal savings.
Why your own figure differs from the official one
Manufacturer figures come from standardised laboratory cycles designed for repeatability, not realism. They use a warm engine, no headwind, no roof box, gentle acceleration and no air conditioning.
Real consumption is dominated by things the test smooths away: cold starts, which can double consumption for the first few kilometres; speed, since air resistance rises with the square of it, so 120 km/h costs far more per kilometre than 90; and stop-start traffic, where you pay to accelerate the car repeatedly and get nothing back.
For budgeting, use the long-term average from your own trip computer rather than the brochure. It already contains your roads, your right foot and your climate.
Working out what a journey actually costs
Normalise everything to litres first, then multiply by the price per litre. Distance ÷ 100 × L/100km gives the litres; litres × price gives the cost.
A 500 km trip at 7 L/100km uses 35 litres, which at 1.60 per litre is 56.00. If your consumption is in MPG, convert it with one of the constants above before doing anything else — mixing units mid-calculation is where most fuel-cost errors come from.
Frequently Asked Questions
Is a lower or higher number better for fuel economy?
It depends on the unit. Lower is better for litres per 100 km, because it measures how much fuel you consume. Higher is better for MPG and km/L, because those measure how far you travel per unit of fuel. They are reciprocals of each other.
What is the difference between US and UK MPG?
Only the size of the gallon. A US gallon is 3.785 litres and an imperial gallon is 4.546, about 20% larger, so the same car scores higher in UK MPG. A car rated 40 MPG in the UK is roughly 33 MPG in US terms.
How do I convert MPG to litres per 100 km?
Divide 235.21 by the US MPG figure, or 282.48 by the UK figure. So 40 US MPG is about 5.9 L/100km and 40 UK MPG is about 7.1 L/100km. The same constants convert back the other way.
Why is improving a thirsty car worth more than improving an efficient one?
Because MPG is a reciprocal, so equal MPG steps are not equal fuel savings. Going from 15 to 25 MPG saves over six times as much fuel across the same distance as going from 45 to 55 MPG. Consumption units like L/100km make this visible; MPG hides it.
Why is my real consumption worse than the official figure?
Official figures come from standardised laboratory cycles with a warm engine, no accessories and gentle acceleration. Cold starts, motorway speeds, air conditioning, roof boxes and traffic all raise real consumption. Use your own trip computer average for budgeting.