L/100km vs MPG
They measure the same thing inversely — litres-per-distance vs distance-per-gallon. The conversions:
MPG(US) = 235.215 ÷ (L/100km) · MPG(UK) = 282.481 ÷ (L/100km)
Note the UK gallon is ~20% bigger than the US gallon, so UK MPG numbers always look higher for the same car. Costs are shown in whatever currency you enter the fuel price in.
FAQ
Why does economy matter for a tuned car?
More power usually means more fuel under load. This makes the trade-off concrete — see the yearly cost change when your consumption goes up by a litre or two.
US or UK gallons?
The calculator shows both. Pick the one that matches where you fill up.