so this shows you can use multiple textures on the same car
Just to avoid misunderstandings: every model can only have one single texture, trust me In other words, if you want to use multiple textures, you have to use multiple models (one texture per model).
because it gives me around 20-30 fps I would need to reduce it down a lot
It's important to keep polycounts as low as possible On the one hand, it has a direct impact on rendering performance (it highly depends on your hardware, an NVIDIA GTX 1080 can definitely handle much more polygons than an integrated Intel HD graphics adapter), usually you can keep "a few million triangles" in mind when it comes to the total polygon count (but keep in mind that the game scene already consists of a lot of triangles).
But on the other hand, high polygon counts also result in bigger file sizes, i.e. especially in multiplayer it takes more bandwith to transfer the model data to the client.
For a decent looking car (if you create a model from scratch), I'd recommend to not use more than 10,000 - 30,000 triangles (you can achieve really great results with these limits, everything above that is basically a waste of resources).