Constant Car Speed using wheel colliders.

I have a scene where there is traffic driving on a stretch off road. First of all I am not sure this is the right approach, but the cars have wheel colliders that I apply a torque on. This gets the cars moving and still interacts with my player car like expected. When crashing into objects the cars still spin their wheels, that is why i am not applying a force to push the car.

The problem is that need the cars to move at a constant speed. 1) How do I apply the torque so that the car will reach and stay at say 50 units/h. 2) Clamping the rigid body's velocity still makes the speed vary a bit depending on the car's torque.+-5units 3) is it even possible to have a object travel at a precise speed using this way ?