Is it a focusing problem rather than a shutter speed problem?
The basic rules I use for RC are,
When the car is moving toward/away from you at speed you need to pre-focus on the spot where you want to take the image, camera should be in 'one-shot' AF mode. I often focus on the spot then flick the lense into manual to lock the focus. The following is pre-focused on the painted track marker.....
When the car is moving but keeping a constant distance from you it's a panning shot, sounds like this is what you are trying to do, it's not easy. I use the 'Servo' AF mode (not sure why its called this, I assume it's 'cos the lense has a servo motor in it
) As long as you can keep the red-dot of the AF point on the car it should be good.
When you talk about Tv vs Av it matters little what you use as long as it's set right fot the light conditions, I use Tv mostly unless light is poor, Jimmy (I think) uses Av a lot more.
It's always difficult if light is moderate or poor. Aperture wide means a tough time focusing, as Mole points out. Aperture too tight and you'll get movement blurr as the shutter speed slows down. You always have to balance the two.