Array Index out of range (JavaScript)

I am making a car game. When I play the game, it freezes on start and tells me that an array index is out of range. The error happens on this line of code in my script: EngineRPM = Mathf.Abs(BackLeftWheel.rpm + BackRightWheel.rpm)/2 * GearRatio[CurrentGear] * DifferentialRatio; EDIT: Here is the full script:

 // 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 DifferentialRatio : float = 3.21;
 var CurrentGear : int = 0;

 var FrontLeftWheel : WheelCollider;
 var FrontRightWheel : WheelCollider;
 var BackLeftWheel : WheelCollider;
 var BackRightWheel : WheelCollider;
 
 // 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 = 600.0;
 var MaxEngineRPM : float = 7000.0;
 var MinEngineRPM : float = 1000.0;
 var EngineRPM : float = 0.0;
 
 //var FrontWheelDrive : int = 1;
 //var RearWheelDrive : int = 1;
 
 // Center Of Mass
 var COMX : float = 0;
 var COMY : float = -0.2;
 var COMZ : float = 0.5;
 
 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.x = COMX;
     rigidbody.centerOfMass.y = COMY;
     rigidbody.centerOfMass.z = COMZ;
 }
 
 function Update () {
     //update center of mass
     rigidbody.centerOfMass.x = COMX;
     rigidbody.centerOfMass.y = COMY;
     rigidbody.centerOfMass.z = COMZ;
     
     // This is to limith the maximum speed of the car, adjusting the drag probably isn't the best way of doing it,
     // but it's easy, and it doesn't interfere with the physics processing.
     rigidbody.drag = rigidbody.velocity.magnitude / 250;
     
     // Compute the engine RPM based on the average RPM of the two wheels, then call the shift gear function
     EngineRPM = Mathf.Abs(BackLeftWheel.rpm + BackRightWheel.rpm)/2 * GearRatio[CurrentGear] * DifferentialRatio;
     if ( EngineRPM>10000) {EngineRPM =10000;}
     if ( EngineRPM<0) {EngineRPM =0;}
     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) + 0.5 ;
     // this line is just to ensure that the pitch does not reach a value higher than is desired.
     if ( audio.pitch > 1.5 ) {
         audio.pitch = 1.5;
     }
 
     // finally, apply the values to the wheels.    The torque applied is divided by the current gear, and
     // multiplied by the user input variable.
     //if (FrontWheelDrive) {
     //    FrontLeftWheel.motorTorque = -EngineTorque * GearRatio[CurrentGear]*DifferentialRatio * Input.GetAxis("Vertical") *1000;
     //    FrontRightWheel.motorTorque = -EngineTorque * GearRatio[CurrentGear]*DifferentialRatio * Input.GetAxis("Vertical") *1000;
     //}
     //if (RearWheelDrive) {
         BackLeftWheel.motorTorque = -EngineTorque * GearRatio[CurrentGear] * DifferentialRatio * Input.GetAxis("Vertical") *100;
         BackRightWheel.motorTorque = -EngineTorque * GearRatio[CurrentGear] * DifferentialRatio * Input.GetAxis("Vertical") *100;
     //}
 
     // the steer angle is an arbitrary value multiplied by the user input.
     FrontLeftWheel.steerAngle = 35 * Input.GetAxis("Horizontal");
     FrontRightWheel.steerAngle = 35 * 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 ( Mathf.Abs(BackLeftWheel.rpm + BackRightWheel.rpm)/2 * GearRatio[i]*DifferentialRatio < MaxEngineRPM ) {
                 AppropriateGear = i;
                 break;
             }
         }
         
         CurrentGear = AppropriateGear;
     }
     
     if ( EngineRPM <= MinEngineRPM ) {
         AppropriateGear = CurrentGear;
         
         for ( var j = GearRatio.length-1; j >= 0; j -- ) {
             if ( Mathf.Abs(BackLeftWheel.rpm + BackRightWheel.rpm)/2 * GearRatio[j]*DifferentialRatio > MinEngineRPM ) {
                 AppropriateGear = j;
                 break;
             }
         }
         
         CurrentGear = AppropriateGear;
     }
 }