How to add car speed to a car controller script

Hello

Im very new to unity and coding
I have a car controller script and would like to able to change the speed the car goes does anyone know a script I can add

so at the moment my code allowd to move foward and back and tern left and right

using UnityEngine;

public class Car : MonoBehaviour
{
    public float maxTorque = 5000000f;

    public Transform centerofMass;

    public WheelCollider[] wheelColliders = new WheelCollider[4];
    public Transform[] tireMeshes = new Transform[4];

    private Rigidbody m_rigidBody;

    void Start()
    {
        m_rigidBody = GetComponent<Rigidbody>();
        m_rigidBody.centerOfMass = centerofMass.localPosition;
    }

    void Update()
    {
        UpdateMeshesPositions();
    }

    void FixedUpdate()
    {
        float steer = Input.GetAxis ("Horizontal");
        float accelrate = Input.GetAxis ("Vertical");

        float finalAngle = steer * 45f;
        wheelColliders [0].steerAngle = finalAngle;
        wheelColliders [1].steerAngle = finalAngle;

        for (int i = 0; i < 4; i++)
        {
            wheelColliders*.motorTorque = accelrate * maxTorque;
        }
    }

    void UpdateMeshesPositions()
    {
        for(int i = 0; i < 4; i++)
        {
            Quaternion quat;
            Vector3 pos;
            wheelColliders*.GetWorldPose(out pos, out quat);

            tireMeshes*.position = pos;
            tireMeshes*.rotation = quat;
        }
    }
}

you multiply your variable “accelrate” by another variable you need to add to your script or change maxtorque

wheelColliders_.motorTorque = accelrate * maxTorque *newVariable;_