Step One
Convert miles to km. The conversion constant is 1.6 km / mile
19 miles * 1.6 km/mile = 30.4 km Notice the miles cancel.
Step Two
Convert the gallons to liters.
This is the sticky part of the question. Where are you? The conversion factor is different in Canada or Great Britain that it is in the United States. If you are not in the US use 4.5 otherwise 1 gallon has 3.78 L in it if it is a US gallon
30.4 km / 3.78 L = 8.04 km per Liter using a US Gallon.
30.4 km / 4.5 L = 6.76 km / Liter using an Imperial Gallon which not used in either Canada or Great Britain. Imperial Gallons are obsolete.