1/40 and 1/60 is impressive, but the good/blurred ratio at the end of the day is not really good enough in most cases. With those speeds it's rather hard to get the car more in focus.
From dragracing which I cover a lot in Santa Pod, when you have fast moving vehicles I always aim to focus at the middle of the car/bike. I use 1 point focus but you could play around with more... the problem you will get is that with AI Servo autofocus, when you'll have more than 1 point, the camera might re-focus on something that will get caught in one of the points and you will loose sharpness on your target
Depends really what is the situation....when you have a head-on situation...naturally you will aim to front bumper...and so on and on...
I think it's a matter of practice. I started off from 1/1000 with compact Olympus ;P and now I'm going down to 1/80, but mostly around 120 as DevilsTower advised. No need to get lower as you will only increase number of photos to delete...unless you don't drink at all and your hand is as stable as Manfrotto tripod ;P (whisper: or you have tons of cash and can afford lenses with image stabilizers)
Hope that helps.
Regards,
Marcin