No Air Driving

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

public class PlayerController : MonoBehaviour
{
    //prevent driving in the air
    [SerializeField] List<WheelCollider> allWheels;
    [SerializeField] int wheelsOnGround;

    private Rigidbody playerRb;

    void FixedUpdate()
    {
        if (IsOnGround())
        {
        //commented out the old code to move forward and replaced with the new line below
        //transform.Translate(Vector3.forward * Time.deltaTime * horsePower * forwardInput);
        //Move the vehicle forward
        playerRb.AddRelativeForce(Vector3.forward * forwardInput * horsePower);
        //have vehicle rotate as it turns
        transform.Rotate(Vector3.up * Time.deltaTime * turnSpeed * horizontalInput);

        //speedometer
        speed = Mathf.Round(playerRb.velocity.magnitude * 3.6f);
        speedometerText.SetText("Speed: " + speed + "kph");

        //rpm
        rpm = Mathf.Round((speed % 30) * 40);
        rpmText.SetText("RPM: " + rpm);
        }
    }

    //prevent driving in the air
    bool IsOnGround()
    {
        wheelsOnGround = 0;
        foreach (WheelCollider wheel in allWheels)
        {
            if (wheel.isGrounded)
            {
                wheelsOnGround++;
            }
        }
        if (wheelsOnGround == 4)
        {
            return true;
        }
        else
        {
            return false;
        }
    }
}