Car Gearbox script

Hi, i’m making a racing game, and i made a car controller script. It works well but i want to add an automatic gearbox to it. How do i do it ?

PS : i want it to shift according to the engine RPM (for example if the engine reaches 100 RPM it switches to second speed) and i want to be able to edit the number of speeds (all the cars in the game doesn’t have the same amount of speeds)

using System.Collections;
using System;
using System.Collections.Generic;
using UnityEngine;

public enum Axel
{
    Front,
    Middle,
    Rear
}

public enum driveType
{
    frontWheelDrive,
    rearWheelDrive,
    allWheelDrive
}

[Serializable]
public struct Wheel
{
    public GameObject model;
    public WheelCollider collider;
    public Axel axel;
}

public enum drivetype
{
    allWheelDrive = 1, rearWheelDrive = 2, frontWheelDrive = 3
}

[RequireComponent(typeof(InputManager))]
public class PlayerCarController : MonoBehaviour
{
    public driveType DriveType;

    public InputManager inputManager;
    public List<Wheel> wheels;
    public Rigidbody localRigidbody;
    public Vector3 centerOfMass;

    //Throttle System variables
    public float torque = 1000f;
    public float maxAcceleration = 20f;
    public float finalMaxAcceleration;
    public float currentSpeed;
    public float topSpeed /*= 200f*/;
    public float topSpeedValue = 200;

    //Steering system variables
    public float turnSensitivity = 1f;
    public float maximumTurnAngle = 45f;

    //Hand brake system variables
    public bool handBrakeIsOn;

    //nitro system variables
    public bool nitroIsOn;
    public float nitroValue = 1;
    
    void Start()
    {
        localRigidbody = GetComponent<Rigidbody>();
        localRigidbody.centerOfMass = centerOfMass;
    }

    private void Update()
    {
        finalMaxAcceleration = maxAcceleration * nitroValue;

        currentSpeed = localRigidbody.velocity.sqrMagnitude;
        //Debug.Log(currentSpeed + " | " + topSpeed);
    }
    
    void FixedUpdate()
    {
        AnimateWheels();
        ThrottleSystem();
        SteeringSystem();
        HandBeakeSystem();
        NitroSystem();
        TopSpeedDefinition();
    }

    void ThrottleSystem()
    {
        foreach (var wheel in wheels)
        {
            if (DriveType == driveType.allWheelDrive)
            {
                wheel.collider.motorTorque = inputManager.throttle * finalMaxAcceleration * torque * Time.deltaTime;
            }
            if (DriveType == driveType.rearWheelDrive)
            {
                if(wheel.axel == Axel.Rear)
                {
                    wheel.collider.motorTorque = inputManager.throttle * finalMaxAcceleration * torque * Time.deltaTime;
                }
            }
            if (DriveType == driveType.frontWheelDrive)
            {
                if (wheel.axel == Axel.Front)
                {
                    wheel.collider.motorTorque = inputManager.throttle * finalMaxAcceleration * torque * Time.deltaTime;
                }
            }
        }
    }

    void TopSpeedDefinition()
    {
        foreach (var wheel in wheels)
        {
            if (currentSpeed > topSpeed)
            {
                wheel.collider.motorTorque = 0f;
            }
            else if (currentSpeed < topSpeed)
            {
                wheel.collider.motorTorque = inputManager.throttle * finalMaxAcceleration * torque * Time.deltaTime;
            }
        }
    }

    void SteeringSystem()
    {
        foreach (var wheel in wheels)
        {
            if (wheel.axel == Axel.Front)
            {
                var _steerAngle = inputManager.steer * turnSensitivity * maximumTurnAngle;
                wheel.collider.steerAngle = Mathf.Lerp(wheel.collider.steerAngle, _steerAngle, 0.5f);
            }
        }
    }

    void HandBeakeSystem()
    {
        if (inputManager.handBrake)
        {
            handBrakeIsOn = true;
        }
        else
        {
            handBrakeIsOn = false;
        }

        foreach (var wheel in wheels)
        {
            if(handBrakeIsOn == true)
            {
                wheel.collider.brakeTorque = 1000000000f;
                wheel.collider.motorTorque = 0f;
            }
            else if(handBrakeIsOn == false)
            {
                wheel.collider.brakeTorque = 0f;
                wheel.collider.motorTorque = inputManager.throttle * finalMaxAcceleration * torque * Time.deltaTime;
            }
        }
    }

    void NitroSystem()
    {
        if(inputManager.nitro)
        {
            nitroIsOn = true;
        }
        else
        {
            nitroIsOn = false;
        }

        if(nitroIsOn == true)
        {
            nitroValue = 5f;
            topSpeed = topSpeedValue * 2f;
        }
        else if (nitroIsOn == false)
        {
            nitroValue = 1f;
            topSpeed = topSpeedValue;
        }
    }

    void AnimateWheels()
    {
        foreach (var wheel in wheels)
        {
            Vector3 wheelColliderPosition;
            Quaternion WheelColliderRotation;
            wheel.collider.GetWorldPose(out wheelColliderPosition, out WheelColliderRotation);
            wheel.model.transform.position = wheelColliderPosition;
            wheel.model.transform.rotation = WheelColliderRotation;
        }
    }
}

using System.Collections;
using System.Collections.Generic;
using UnityEngine;

public class Test : MonoBehaviour
{
    // The RPMs
    public int RPM;
    public int MaxRPM;
    public int ChangeRPM;
    // This is the actual GearBox
    public float[] Gearbox;
    public int CurrentGear;
    public bool CanShiftGear = false;
    // The vehicle Speed
    public float Speed;
    public Rigidbody RB;
    public float Torque;
    private void Update()
    {
        // It checks if the RPMs are the desired ones and if you can shift
        // UpShift
        if (RPM >= ChangeRPM && CanShiftGear && CurrentGear != Gearbox.Length -1)
        { CurrentGear++; CanShiftGear = false; }
        //DownShift
        if(RPM <= 2000f && CanShiftGear && CurrentGear != 0)
        { CurrentGear--; CanShiftGear = false; }
        // Takes Car Of the boolean
        if(RPM < ChangeRPM)
        { CanShiftGear = true; }
        // Once you have reached the last gear ypu can not shift
        if(CurrentGear == Gearbox.Length -1)
        { CanShiftGear = false; }
        // Speed
        Speed = RB.velocity.magnitude;
        GearBox();
    }
    void GearBox()
    {
        if(Speed >= Gearbox[CurrentGear])
        { Torque = 0f; }
        else
        { Torque = 1f; }
    }
}

This should work.
For the Top Speed just go into the inspector and edit the size and floats of the GearBox and fill each gap with wathever top speed you want in that gear.
Also for the RPMs of the car I would recommed smoething like the WheelsCollider.rpm