I want to build a car using wheel collider, mesh collider and rigidbody. The problem is that unity wants the meshcollider to be convex, and that cannot happen because my base model is bigger than 255 poly's.
You should consider using a separate, lower-poly mesh for your mesh collider. Not only will it fix your problem (if you can manage a mesh that's lowpoly enough to be convex), but it will also make physics calculations much less expensive.
If your mesh has concave shapes that you need to be reflected through your colliders, you can split your mesh up in your 3D modeling program of choice and generate multiple collider meshes, then use multiple mesh colliders.
There are even commercial Asset Store tools that do this procedurally., but it should be trivial to do in any 3D modeling software.