Like the other guys have said, poly count really doesn't have an effect on performance. The biggest performance killer is going to be as @Sniper296 stated, textures. But also the amount of materials and textures used in the vehicle. They use multiple materials, that use the same detail texture, but use different specular textures, and such. This can be avoided by using 1 specular map texture, that has multiple specular colors on it. This also applies to the normal (bump) map. The other issue is that each piece of equipment has it's own set of textures, and own set of materials (sometimes duplicating).
Here is an example of a vehicle's equipment being optimized. These are the parts that I have in my Crown Vic. Now my CVPI has a spotlight, camera, radio, and siren controller, along with the center console and stand. Each part, if not optimized would have it's own texture, own specular color (most times multiple materials for different spec maps) etc. Instead, I use only 3 materials. One for the normal textures, one for the glass on the spotlight, and one for the emissives. BUT....all three use the same exact texture. Doing it this way eases the load on the GPU and it doesn't have to search in it's memory for the texture, and try to remember which material it goes to.