How to make about?

I mean if my car.rotation.z is about 90 so like 89.999 or 91.03 etc.
set velocity 0?
I hope somebody understand my question. Sorry for my english

You should not be accessing the individual components of a rotation (x,y,z,w). They are Quaternions, and non-intuitive. Assuming car is a transform, you can access car.eulerAngles. But there are some gotchas with this as well. For example, you can set the eulerAngles to (180,0,0) and immediately read them back and get (0,180,180), which is the same physical rotation in a different eulerAngle representation. As for your question, you can do:

if (Mathf.Abs(car.eulerAngles.z - 90) < some_small_value) {
  //Do something
}