Different top speed when going downhill

Hello!

I need my car to have a different top speed when going downhill. How do I implement this?

public WheelCollider wheelFL;
public WheelCollider wheelFR;
public WheelCollider wheelBL;
public WheelCollider wheelBR;
public float maxTorque = 50;
public float decelerationSpeed = 30;
public float currentSpeed = 0;
public float topSpeed = 600;

    void Control ()
    		{
    				// Top speed & current speed
    				currentSpeed = 2 * 22 / 7 * wheelBL.radius * wheelBL.rpm * 60 / 100;
    				currentSpeed = Mathf.Round (currentSpeed);
    
    				if (currentSpeed <= topSpeed && currentSpeed > -maxReverseSpeed && !braked) {
    						wheelBR.motorTorque = maxTorque * Input.GetAxis ("Vertical");
    						wheelBL.motorTorque = maxTorque * Input.GetAxis ("Vertical");	
    				} else {
    						wheelBR.motorTorque = 0;
    						wheelBL.motorTorque = 0;
    				}
    
    		}

Probably check the velocity of the rigidbody, if its y component is < 0 it is going downhill?