Why the car gears dosen't work.

I have a big problem with my car script , the gears is not working . I want a automatic gearbox but my function who change gears dosent want to update the gears just if I call in in the update function. But when I call in in the update the gear jump to 4th gear not in to 2nd.(he jump at the last gear from array)

 using System.Collections;
 using System.Collections.Generic;
 using UnityEngine;
 using UnityEngine.UI;
 
 
 
 
 
 public class CarController : MonoBehaviour
 {
 
 
     public AnimationCurve EnginePower;
     public Button upshift;
 
     public steeringwheel2 Steering;
     public GameManager Manager;
     public Slider slider;
 
     public Rigidbody car_rb;
     public double currentspeed;
 
     public int[] gearSpeeds = new int[] { 40, 80, 120, 160, 220 };
     public float[] gears;
     public int gear = 1;
     [HideInInspector] public bool reverse = false;
 
     public float m_horizontalInput;
     public float m_verticalInput;
     public bool isbrake = false;
 
 
     public float DownForceValue = 10f;
 
     public float wheelsRPM;
     public float totalpower;
     public float maxRPM;
     public float engineRPM;
 
     
 
 
     public void Steer()
     {
         m_steeringAngle = Steering.GetAngle() / maxSteerAngle;
         frontDriverW.steerAngle = m_steeringAngle;
         frontPassengerW.steerAngle = m_steeringAngle;
     }
 
     public void Accelerate()
     {
 
         if (currentspeed <= 220)
         {
 
 
             if (m_verticalInput == 1)
             {
                 frontDriverW.motorTorque = totalpower;
                 frontPassengerW.motorTorque = totalpower;
                 rearDriverW.motorTorque = totalpower;
                 rearPassengerW.motorTorque = totalpower;
             }
             else if (m_verticalInput == 0)
             {
                 frontDriverW.motorTorque = 0;
                 frontPassengerW.motorTorque = 0;
                 rearDriverW.motorTorque = 0;
                 rearPassengerW.motorTorque = 0;
             }
         }
     }
 
     public void If()
     {
         if (m_verticalInput == -1)
         {
             Brake();
         }
         else if (m_verticalInput == 0)
         {
             NotBrake();
         }
     }
 
     public void Brake()
     {
 
 
         frontDriverW.brakeTorque = brakePower;
         frontPassengerW.brakeTorque = brakePower;
         rearDriverW.brakeTorque = brakePower;
         rearPassengerW.brakeTorque = brakePower;
     }
 
     public void NotBrake()
     {
         frontDriverW.brakeTorque = 0;
         frontPassengerW.brakeTorque = 0;
         rearDriverW.brakeTorque = 0;
         rearPassengerW.brakeTorque = 0;
     }
 
    
     public void shifter()
     {
         if (engineRPM > maxRPM && gear < gears.Length - 1)
         {
             gear++;
         }
     }
 
 
     private void UpdateWheelPoses()
     {
         UpdateWheelPose(frontDriverW, frontDriverT);
         UpdateWheelPose(frontPassengerW, frontPassengerT);
         UpdateWheelPose(rearDriverW, rearDriverT);
         UpdateWheelPose(rearPassengerW, rearPassengerT);
     }
 
     private void UpdateWheelPose(WheelCollider _collider, Transform _transform)
     {
         Vector3 _pos = _transform.position;
         Quaternion _quat = _transform.rotation;
 
         _collider.GetWorldPose(out _pos, out _quat);
 
         _transform.position = _pos;
         _transform.rotation = _quat;
 
 
     }
     public void Calculatingenginepower()
     {
         WheelRpm();
         
         
         float velocity = 0.0f;
         totalpower = EnginePower.Evaluate(engineRPM) * (gears[gear]);
         engineRPM = Mathf.SmoothDamp(engineRPM, 1000 + (Mathf.Abs(wheelsRPM) * 3.6f * (gears[gear])), ref velocity, 0.09f);
 
 
         
     }
     public void WheelRpm()
     {
         wheelsRPM = frontDriverW.rpm + frontPassengerW.rpm + rearDriverW.rpm + rearPassengerW.rpm / 4;
 
 
 
     }
 
     public void DownForce()
     {
         car_rb.AddForce(-transform.up * DownForceValue * car_rb.velocity.magnitude);
     }
     public void Update()
     {
         Steer();
         UpdateWheelPoses();
         Accelerate();
         Calculatingenginepower();
         DownForce();
         WheelRpm();
         
         currentspeed = car_rb.velocity.magnitude * 3.6;
     }
 
     
 
 
 
 
 
 
     private float m_steeringAngle;
 
 
     public WheelCollider frontDriverW, frontPassengerW;
     public WheelCollider rearDriverW, rearPassengerW;
     public Transform frontDriverT, frontPassengerT;
     public Transform rearDriverT, rearPassengerT;
     public float maxSteerAngle = 30;
     public float motorForce = 500;
     public float brakePower = 100000;
 
 }