I have the following categories
Car parts (0) Tires (0) Rims (5)
Now in the product listing obviously the car parts en tires listing returns 0 products. But for the user this is ankward. Is it possible to show child items if a category is empty?
Have you marked the category as an anchor category in the administrative interface? That should allow child categories' products to show up in the parent category listing.
Hope that helps.