How add kilometre per hour system in car script,how to add kilometre per hour speed system in script

I want my car to move 0 KPH TO 60 KPH ( when i press "W" the speed increase 0 to 60 KPH) and after reaching 60 KPH car continues to move on 60 KPH my script is this

public class Carcontroller3 : MonoBehaviour { [SerializeField] private WheelCollider FrontLeftWheelC; [SerializeField] private WheelCollider FrontRightWheelC; [SerializeField] private WheelCollider RearLeftWheelC; [SerializeField] private WheelCollider RearRightWheelC;

 [SerializeField] private Transform FrontLeftWheelT;
 [SerializeField] private Transform FrontRightWheelT;
 [SerializeField] private Transform RearLeftWheelT;
 [SerializeField] private Transform RearRightWheelT;

 private float currentSteerAngle;

 private const string HORIZONTAL = "Horizontal";
 private const string VERTICAL = "Vertical";
 [SerializeField] private float maxTorque = 200f;
 [SerializeField] private float SteerAngle = 30f;
 [SerializeField] private float Speedinkph = 100;

 private float horizontalInput;
 private float verticalInput;

 public void FixedUpdate()
 {
     GetInput();
     Acceleration();
     HandleSteering();
     UpdateWheel();
 }

 public void GetInput()
 {
     horizontalInput = Input.GetAxis("Horizontal");
     verticalInput = Input.GetAxis("Vertical");
 }

 public void Acceleration()
 {
     RearLeftWheelC.motorTorque = verticalInput * maxTorque;
     RearRightWheelC.motorTorque = verticalInput * maxTorque;
 }

 public void HandleSteering()
 {
     currentSteerAngle = SteerAngle * horizontalInput;
     FrontLeftWheelC.steerAngle = currentSteerAngle;
     FrontRightWheelC.steerAngle = currentSteerAngle;
 }

 public void UpdateWheel()
 {
     UpdateSingleWheel(FrontLeftWheelC, FrontLeftWheelT);
     UpdateSingleWheel(FrontRightWheelC, FrontRightWheelT);
     UpdateSingleWheel(RearLeftWheelC, RearLeftWheelT);
     UpdateSingleWheel(RearRightWheelC, RearRightWheelT);
 }

 public void UpdateSingleWheel(WheelCollider WheelCollider, Transform WheelTransform)
 {
     Vector3 pos;
     Quaternion rot;
     WheelCollider.GetWorldPose (out pos, out rot );
     WheelTransform.position = pos;
     WheelTransform.rotation = rot;
 }

}

please help me out