How to get a smooth average of wheel collider RPM;

In your case please refer this JS script. Gear shifting also explain here. I found this from Unity forum. I downloaded by here

 // ----------- CAR TUTORIAL-----------------
 
 // These variables allow the script to power the wheels of the car.
 var FrontLeftWheel : WheelCollider;
 var FrontRightWheel : WheelCollider;
 
 // These variables are for the gears, the array is the list of ratios. The script
 // uses the defined gear ratios to determine how much torque to apply to the wheels.
 var GearRatio : float[];
 var CurrentGear : int = 0;
 
 // These variables are just for applying torque to the wheels and shifting gears.
 // using the defined Max and Min Engine RPM, the script can determine what gear the
 // car needs to be in.
 var EngineTorque : float = 230.0;
 var MaxEngineRPM : float = 3000.0;
 var MinEngineRPM : float = 1000.0;
 private var EngineRPM : float = 0.0;
 
 
 
 function Start () {
     // I usually alter the center of mass to make the car more stable. I'ts less likely to flip this way.
 rigidbody.centerOfMass += Vector3(0, -1, .25);
     }
 
 function Update () {
     
         // Compute the engine RPM based on the average RPM of the two wheels, then call the shift gear function
     EngineRPM = (FrontLeftWheel.rpm + FrontRightWheel.rpm)/2 * GearRatio[CurrentGear];
     ShiftGears();
 
     // set the audio pitch to the percentage of RPM to the maximum RPM plus one, this makes the sound play
     // up to twice it's pitch, where it will suddenly drop when it switches gears.
     audio.pitch = Mathf.Abs(EngineRPM / MaxEngineRPM) + 1.0 ;
     // this line is just to ensure that the pitch does not reach a value higher than is desired.
     if ( audio.pitch > 2.0 )
     {
         audio.pitch = 2.0;
     }
 
     // finally, apply the values to the wheels.    The torque applied is divided by the current gear, and
     // multiplied by the user input variable.
     FrontLeftWheel.motorTorque = EngineTorque / GearRatio[CurrentGear] * Input.GetAxis("Vertical");
     FrontRightWheel.motorTorque = EngineTorque / GearRatio[CurrentGear] * Input.GetAxis("Vertical");
         
     // the steer angle is an arbitrary value multiplied by the user input.
     FrontLeftWheel.steerAngle = 10 * Input.GetAxis("Horizontal");
     FrontRightWheel.steerAngle = 10 * Input.GetAxis("Horizontal");
 }
 
 function ShiftGears() {
     // this funciton shifts the gears of the vehcile, it loops through all the gears, checking which will make
     // the engine RPM fall within the desired range. The gear is then set to this "appropriate" value.
     if ( EngineRPM >= MaxEngineRPM ) {
         var AppropriateGear : int = CurrentGear;
         
         for ( var i = 0; i < GearRatio.length; i ++ ) {
             if ( FrontLeftWheel.rpm * GearRatio[i] < MaxEngineRPM ) {
                 AppropriateGear = i;
                 break;
             }
         }
         
         CurrentGear = AppropriateGear;
     }
     
     if ( EngineRPM <= MinEngineRPM ) {
         AppropriateGear = CurrentGear;
         
         for ( var j = GearRatio.length-1; j >= 0; j -- ) {
             if ( FrontLeftWheel.rpm * GearRatio[j] > MinEngineRPM ) {
                 AppropriateGear = j;
                 break;
             }
         }
         
         CurrentGear = AppropriateGear;
     }
 }