US and UK mpg are not the same figure — a UK gallon is 20% larger, so UK mpg always looks better.
L/100km falls as efficiency improves; km/L and mpg rise. They move in opposite directions.
Real consumption is usually 10-20% worse than the manufacturer's figure, and worse again in cold weather, in traffic, or above 110 km/h.
Currency
Amounts follow the currency selected above.
How do I work out fuel cost for a trip?
Divide the distance by your fuel efficiency to get the fuel needed, then multiply by the price per litre or gallon.
Is US mpg the same as UK mpg?
No. A UK gallon is 4.546 litres against the US 3.785, so the same car shows about 20% higher mpg on UK figures.
Why does my real economy differ from the official figure?
Official tests are laboratory cycles. Cold starts, short journeys, traffic, roof racks, air conditioning and speeds above 110 km/h all reduce it, typically by 10-20%.
How do I convert mpg to L/100km?
First convert mpg to km/L (× 0.4251 for US mpg, × 0.3540 for UK mpg), then divide 100 by that figure. Or just pick "mpg (US)" or "mpg (UK)" as the efficiency unit above and the conversion happens automatically.