Hi!
I tried to get a formula for calculating max velocity of a rigidbody, but after couple of days of trial and error, where I tried to apply some physics formulas to find my values I got nothing because I was unable to figure out how unity calculates all of this.
I got some results from this formula found on the forums(without gravity and friction):
maxVelocity = ((force.magnitude / drag ) - fixedDeltaTime * force.magnitude )/ mass;
and force is just:
Vector3.forward * mass * acceleration;
Right now I need to set the acceleration to achieve max speed, but can’t quite get the formula right for this. Is it even possible? I have tried transforming this formula, but I failed 
(the shame!)
This is for a rigidbody without gravity and any friction.
some code:
public Rigidbody r;
public float acceleration = 1f;
public float currentForwardSpeed ;
private Transform _t;
private float force;
void Start()
{
_t = GetComponent<Transform>();
acceleration = ???????
force = _t.forward * r.mass * acceleration;
}
void FixedUpdate)
{
currentForwardSpeed = _transform.InverseTransformDirection(_rigidbody.velocity).z;
if(Input.GetKey(KeyCode.W))
{
float maxVelocity = ((force.magnitude / r.drag ) - fixedDeltaTime * force.magnitude )/ r.mass;
}
}