Litres to gallons
One litre is 0.264172 US gallons, because a US liquid gallon is defined as exactly 3.785411784 litres. The imperial gallon used in the UK is larger, at 4.54609 litres, so the same number of litres is only 0.219969 imperial gallons. About seventeen per cent fewer.
How to convert litres to gallons
Fuel economy is where the two gallons cause real trouble. Thirty miles per US gallon is 36 miles per imperial gallon, and 7.84 litres per 100 km. A car advertised at "40 mpg" in Britain is not the same car as one advertised at 40 mpg in the United States, and neither figure converts to the other by simple arithmetic.
Questions
The US liquid gallon, 3.785411784 litres. It is the one meant whenever a gallon is quoted in the United States.
4.54609 litres, about 20% more than a US gallon. It was originally defined as the volume of ten pounds of water.
A separate unit of 4.40488 litres, used for grain and produce. It appears rarely, and never for liquids.
A petroleum barrel is 42 US gallons, which is 158.99 litres. Other barrels; beer, for instance. Are different sizes again.