One of the best seperators between real cars and toy cars, is in my experience the shaders.
I've spent quite some time with BMW, where we needed to hit the exact glossyness, reflection and feeling of one of their chrome textures.
Without it, the car looked like a small plastic toy, but once it got on, the whole imagry changed, and the car turned into a "believable" vehicle.
Also, use of camera is important here as well. Notice in both car games and especially car ads, how they portray the car to make it look big and robust. In reality, cars aren't that impressive in size, but they always manage to make that feel to it.
And last, but not least, make sure it FEELS like a car. The audio, the steering, the way it moves in place when you hit the brakes. The feel of an object is the final touch in making it feel believable.