Player car controller for Unity3d supporting multiple axles.

CarController.cs

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

public class CarController : MonoBehaviour
{
    public List<AxleInfo> AxleInfos;
    public float MaxMotorTorque;
    public float MaxSteeringAngle;

    public Transform CenterOfMass;

    void FixedUpdate()
    {
        this.GetComponent<Rigidbody>().centerOfMass = CenterOfMass.localPosition;

        float motor = MaxMotorTorque * Input.GetAxis("Vertical");
        float steering = MaxSteeringAngle * Input.GetAxis("Horizontal");

        foreach(var info in AxleInfos)
        {
            if (info.IsSteering)
            {
                if (info.InvertSteering)
                    steering *= -1;

                info.LeftWheel.steerAngle = steering;
                info.RightWheel.steerAngle = steering;
            }

            if (info.IsMotor)
            {
                info.LeftWheel.motorTorque = motor;
                info.RightWheel.motorTorque = motor;
            }

            ApplyLocalWheelPosition(info.LeftWheel, info.LeftWheelTransform);
            ApplyLocalWheelPosition(info.RightWheel, info.RightWheelTransform);
        }
    }

    void ApplyLocalWheelPosition(WheelCollider wheel, Transform t)
    {
        Vector3 position;
        Quaternion rotation;

        wheel.GetWorldPose(out position, out rotation);

        t.position = position;
        t.rotation = rotation;
    }
}

[System.Serializable]
public class AxleInfo
{
    public WheelCollider LeftWheel;
    public WheelCollider RightWheel;

    public Transform LeftWheelTransform;
    public Transform RightWheelTransform;

    public bool IsMotor;
    public bool IsSteering;
    public bool InvertSteering;
}