A detailed 3D object may look nice, but it takes longer for the game to work out how a car should respond in the event of a collision than it would for a simple object. A simple object would not have those issues, but it wouldn't look as good.
You can have the best of both worlds by making sure the detailed object cannot be collided with, and then place an invisible solid object over it.
This trye wall has around 2500 polygons, because each stack of tyres has 8 sides.
The collision object, which will not be rendered in game, can be much simpler. This object only has 10 polygons.
Addition: Use smaller face areas. The collision detection algorithm won't work well if the distance to nearest edge is too great. In this example 5 panels lengthwise would be recommended increasing the total polycount to 20.
This image demonstrates the two different collision types, edge-to-edge and edge-to-face.. Due to performance reasons the full algorithm scope can not be used in game so there's a limit of how big individual face can be before the collision is not registered.