The average miles per gallon(mpg) of US vehicles in the year 2017 was 24.9 miles. Not spectacular but it is a base. Every vehicle and country will have different answers.
Since you asked for kilometers per liter, some conversions will be necessary.
The following conversion factors will be useful.
1 gallon = 3.785 liters
1 mile = 1.609 km
Part 1.
Using the US average mpg = 24.9 miles, lets convert this to kilometers.
24.9 miles x 1.609 km/mile = 24.9 x 1.609 km =
40.06 km
Part 2.
Now find the kilometers per liter of petrol
40.06 km /3.785 liters ≈ 10.58 km/l
Double Check: recalculated
Answer: Average Kilometers per Liter of petrol for US vehicles in 2017 was 10.58 km/l