[THE MORE YOU KNOW WITH ANTAWIRYA]
Shaking cars after filling up fuel is meant to move the fuel to empty spots in the tank, so that the air in tank would exit from it. This condition would make the fuel to ne more congested.
This statement is a myth. The trick is totally false because it’s not in accordance with the physics law. Liquds like the fuel will automatically fill in the empty space in the container which is the tank. So, there’s no need to shake the car to get the optimal result.