Bulk space is a unit of measurement to show the amount of free space available in a vehicle or lockup space in town or camp lockup buildings.
All weapons, ammo and resources (food, water, fuel or other commodities) have a certain bulk applied to the space they take up. While tires that are installed on your vehicle takes up none of the available space IN the vehicle, if you carry tires in your cargo space, they will take up a certain amount of bulk space.
Armor does NOT take up any bulk inside a chassis. Details about armor are available on the armor page.