The turismo has ~7000 triangles (or ~3500 quads)
The mustang bullitt ~3600 tri's (~1200 quads)
The Semi ~11000 tri's (~5500 quads)
Those are all old, amateurishly, and inefficiently made meshes though. I have a much cleaner, unfinished car at 3000 tri's that looks more detailed than what I have made in the past.
The default jeep is ~600 tri's I think.
I guess just try to not go more than 4000 quads if you can. But I'm sure somebody like Barnabas would have a totally different take on recommended vehicle face counts. The fewer faces the better, but obviously more detail yields more faces.