This is the same silver metallic carpaint shader that I made for my porsche (this and this)
I added some experimental dirt, not tested on an actual car but just delete the dirt layer if you don't want it.
NOTE: The flakes are tiled to fit a UVW map of 100x100x100cm on map channel 2. Add a 100x100x100cm UVW box-map set to map channel 2 on your car models and it will look good without messing up exisitng UVW maps.
Scene with shader ball and high res HDRI is included. Flake texture by Mr. Jeff Patton.
Download link: