Two gallons, two directions, one honest comparison
Convert MPG to litres per 100 km and back, with US and imperial gallons handled separately. Shows all four figures at once.
Fuel economy is the conversion most likely to mislead, for two reasons. American and British gallons are different sizes, and L/100 km runs backwards to MPG — lower is better on one scale, higher on the other.
Because it is an inverse relationship, you cannot convert it in your head with a simple factor. This page shows L/100 km, km/L, US MPG and imperial MPG together so the comparison is unambiguous.
All four economy figures at once from a single input US and UK MPG kept separate, since the imperial gallon is 20% larger km/L for markets that quote distance per litre US gallon = 3.785 litres, imperial gallon = 4.546 litres. Manufacturer figures come from standardised test cycles; real-world consumption is typically 10–20% worse. Is 30 MPG the same in the US and the UK? No. 30 US MPG is 7.84 L/100 km, while 30 imperial MPG is 9.42 L/100 km. The British figure describes a noticeably thirstier car.
Why can't I just divide to convert MPG? Because the two scales are inverses of each other — distance per fuel versus fuel per distance. You divide into a constant rather than multiplying by one.
Which unit is actually more useful? L/100 km, for a subtle reason: it scales linearly with fuel cost. Going from 15 to 14 L/100 km saves far more fuel than going from 6 to 5 MPG suggests.
How do I work out my real consumption? Fill the tank, reset the trip meter, drive normally, then refill. Litres added divided by kilometres travelled, times 100, gives your true L/100 km.
Worked example A European car quoted in MPG
Input: 8 L/100 km
Output: 29.4 MPG (US) · 35.3 MPG (UK) · 12.5 km/L
Note: The same car looks nearly six MPG thriftier in British adverts purely because the gallon is bigger.
