They're these wheels. They're great for cars.
My suggestion would be to try building the same car in a number of different sizes, to see which one looks best to you. Maybe try 4, 5 and 6 studs wide?
I'd consider all my vehicles to be of a similar scale. I have vehicles in 4, 5, 6, 7, 8, 9 and 10 wide, so it really does depend on the vehicle in question.