How to Use This Tool
Fuel economy is one of the few everyday numbers where two countries use the same word, the same abbreviation, and different units. An American car rated at 30 MPG would be advertised as 36 MPG in Britain, because the imperial gallon is 4.546 litres against the US gallon's 3.785. Same car, same fuel, 20% difference in the headline number.
This converter keeps the two apart and always shows both, so you can compare a US road test against a UK one without quietly concluding that the British version is more efficient.
The other trap: the scale runs backwards
MPG and km/L are efficiency measures — more is better. L/100 km is a consumption measure — less is better. Converting between them means taking a reciprocal, not multiplying, which is why the relationship is a curve rather than a straight line.
That curve has a consequence people consistently get wrong. Going from 15 to 20 MPG saves far more fuel than going from 40 to 50 MPG, even though the second improvement looks bigger.
This is the reason Europe switched to L/100 km and why fleet managers think in it. Consumption per distance is linear in fuel, so the savings are visible directly. If you are choosing between two upgrades, compare the L/100 km figures, not the MPG ones.
Using the cost panel
Enter your fuel price and a distance, and the panel shows what that trip costs and what a year of similar driving would cost. The price unit is separate from the consumption unit on purpose — plenty of people know their consumption in MPG and their fuel price per litre, and doing that conversion in your head is where the arithmetic usually goes wrong.
Honest limits
Official figures are laboratory results. Real consumption depends on speed, temperature, tyre pressure, load, terrain and how you drive, and typically comes in 10–25% worse than the sticker. Use your own measured figure if you have one: fill the tank, drive, refill, and divide. That number is worth more than any manufacturer's.