MPG to L/100km: How to Read Fuel Economy in Any Unit
Convert MPG to L/100km and back, tell US from UK gallons, and understand why fuel economy is inverse not linear — with worked numbers for car buying.
MPG to L/100km: How to Read Fuel Economy in Any Unit
I shop for cars across two markets, and for the longest time I made the same dumb error. A German review would praise a hatchback at 5.5 L/100km, an American one would brag about 42 MPG, and I would try to judge them side by side by feel. They were often the same car. The numbers only look unrelated because fuel economy hides three different traps at once: an inverse relationship, two sizes of gallon, and a unit that counts up where the others count down. Once you see all three, the conversions stop feeling like guesswork.
Distance per fuel versus fuel per distance
MPG and L/100km do not just use different words for the same idea. They measure opposite things.
MPG asks: how far can I drive on a fixed amount of fuel? More miles is better. Its cousins km/L and mi/L work the same way — bigger is greener. L/100km flips the question: how much fuel do I burn over a fixed distance? Fewer litres is better. A car at 5 L/100km sips less than one at 9 L/100km, so here the smaller number wins.
This is the single most common mistake in a car forum thread. Someone posts two figures in two units, picks the bigger one as the winner, and is wrong half the time. If you only remember one rule from this post: convert both numbers to the same unit before you compare. Reading "32 MPG vs 7 L/100km" and eyeballing it is how people talk themselves into the worse car.
Why the relationship is inverse, not linear
Because one unit is fuel-over-distance and the other is distance-over-fuel, converting between them is a reciprocal, not a multiplication. For US MPG the shortcut is:
L/100km = 235.215 ÷ MPG
That division has a consequence most people miss: equal steps in one unit are not equal steps in the other. Going from 20 to 30 MPG saves a real chunk of fuel — 11.76 down to 7.84 L/100km, almost 4 litres per 100 km. Going from 40 to 50 MPG, the same 10-MPG jump, only drops you from 5.88 to 4.70 L/100km, barely over 1 litre. The higher your MPG already is, the less each extra mile-per-gallon actually saves at the pump.
That is why European labels favour L/100km and why fuel-saving math gets argued about endlessly. A salesperson loves to quote the MPG jump because the number looks dramatic; the litres you actually save tell a flatter, more honest story at the top end.
A worked example: 8 L/100km
Say a review lists a sedan at 8 L/100km and you think in MPG. Run the reciprocal:
235.215 ÷ 8 = 29.4 MPG (US)
So 8 L/100km is about 29.4 MPG — solidly mid-pack for a petrol sedan, nothing exotic. Going the other direction is the same arithmetic in reverse: a 30 MPG (US) car lands at 235.215 ÷ 30 ≈ 7.84 L/100km, and 6 L/100km comes back as 235.215 ÷ 6 ≈ 39 MPG. The fuel-consumption-converter does this reciprocal automatically and shows all five units at once, so you never have to keep the magic number in your head or remember which way to divide.
There is also one clean crossover worth memorising: km/L and L/100km multiply to 100, so 10 km/L is exactly 10 L/100km — the one point where they meet. Above it they split: 20 km/L is 5 L/100km, 4 km/L is 25 L/100km. Doubling your km/L always halves your L/100km.
The US gallon and the UK gallon are not the same
Here is the trap that has cost me real money in a brochure comparison. A US gallon is 3.785 litres. An Imperial (UK) gallon is 4.546 litres — about 20 percent larger. MPG counts miles per gallon, so a bigger gallon yields more miles, and the same car reads higher in UK MPG than in US MPG.
A car listed at 30 MPG in the US is roughly 36 MPG in the UK. The fuel use is identical; only the bucket changed size. So when a used-car listing brags about "70 MPG," your first question is which gallon. 70 US MPG is about 3.36 L/100km — hybrid or small-diesel territory. Quietly quoted as UK MPG, the same 70 is a far more ordinary number. Sellers know this, and an inflated spec sheet often just swaps gallons and hopes you do not notice. When a US and a UK driver compare notes, the cleanest fix is to drop gallons entirely and convert both to L/100km or to mi/L, which sidesteps the gallon question altogether.
Putting it to work when you buy a car
Conversions are not an academic exercise — they change which car you sign for. Three habits I use now:
First, normalise every figure to one unit before I compare anything. Brochure says 32 US MPG, review says 38 UK MPG? Both become L/100km (about 7.35 and 7.43), and the tiny gap reveals it is one car measured two ways, not a real efficiency difference.
Second, I check whether a claimed economy is even physically plausible. A big petrol SUV advertised at 3.4 L/100km is a typo or a lie; a small diesel hatch at that figure is believable.
Third, I translate economy straight into money, because L/100km is only half the decision. Knowing a car burns 8 L/100km tells you nothing about your annual bill until you fold in your mileage and local fuel price — that is what the fuel-cost-calculator is for. A 1 L/100km difference between two finalists can be a rounding error or a few hundred dollars a year, depending entirely on how far you drive.
Get those three habits down and the unit games stop working on you. The car that looks best in the headline number is often not the car that is best per litre — and the only way to know is to read every economy figure in a unit your gut already trusts.
Made by Toolora · Updated 2026-06-13