public float gear1 = 1; //значение подобрать методом тыка
public float gear2 = 0.8f; //значение подобрать методом тыка
public float gear3 = 0.6f; //значение подобрать методом тыка
public float gear4 = 0.4f; //значение подобрать методом тыка
public float gear5 = 0.2f; //значение подобрать методом тыка
public float currentGearForce;
public int currentGear = 0; //0== нейтралка.

void Update()
{
    if (Input.GetKeyDown(Keycode.W) && currentGear != 0){
        Умножаем скорость машины на переменную currentGearForce * Time.deltaTime;
    }
    if (currentGear == 1){
        currentGearForce = gear1;
    }
    if (currentGear == 2){
        currentGearForce = gear2;
    }
    if (currentGear == 3){
        currentGearForce = gear3;
    }
    if (currentGear == 4){
        currentGearForce = gear4;
    }
    if (currentGear == 5){
        currentGearForce = gear5;
    }
}

currentGear ++;

...
  void Update ()
  {
   if (spinUpdateRate == UpdateRate.Disabled)
   {
    foreach (WheelData wd in m_wheelData)
    {
     UpdateSteering(wd);
    }
   }
   else
    if (spinUpdateRate == UpdateRate.OnUpdate || wheelPositionMode == PositionMode.Accurate)
   {
    bool needDisableColliders =    m_rigidbody.interpolation != RigidbodyInterpolation.None
     && wheelPositionMode == PositionMode.Accurate;
    
    if (needDisableColliders)
     DisableCollidersRaycast();
    
    foreach (WheelData wd in m_wheelData)
    {
     UpdateSteering(wd);
     UpdateTransform(wd);
    }
    
    if (needDisableColliders)
     EnableCollidersRaycast();
   }
   
   // Drag state is smoothly faded to zero. It gets raised/modified from the drag contacts.
   
   if (processContacts)
   {
    UpdateDragState(Vector3.zero, Vector3.zero, m_localDragHardness);
    // debugText = string.Format("Drag Pos: {0}  Drag Velocity: {1,5:0.00}  Drag Friction: {2,4:0.00}", localDragPosition, localDragVelocity.magnitude, localDragFriction);
   }
  }
  
  
  void FixedUpdate ()
  {
   // Ensure input values within range
   
   throttleInput = Mathf.Clamp (throttleInput, -1.0f, +1.0f);
   brakeInput = Mathf.Clamp01(brakeInput);
   handbrakeInput = Mathf.Clamp01(handbrakeInput);
   
   // Update common variables
   
   m_speed = Vector3.Dot(m_rigidbody.velocity, m_transform.forward);
   m_speedAngle = Vector3.Angle(m_rigidbody.velocity, m_transform.forward) * Mathf.Sign(m_speed);
   
   // Prepare common data
   
   float referenceDownforce =
    computeExtendedTireData? (m_rigidbody.mass * Physics.gravity.magnitude) / m_wheelData.Length : 1.0f;
   
   // Apply wheel physics
   
   bool needUpdateVisuals =
    spinUpdateRate == UpdateRate.OnFixedUpdate && wheelPositionMode == PositionMode.Fast;
   
   int groundedWheels = 0;
   
   foreach (WheelData wd in m_wheelData)
   {
    if (!disallowRuntimeChanges)
     UpdateWheelCollider(wd.collider);
    
    if (needUpdateVisuals)
     UpdateSteering(wd);
    
    UpdateSuspension(wd);
    UpdateLocalFrame(wd);
    UpdateGroundMaterial(wd);
    
    ComputeTireForces(wd);
    ApplyTireForces(wd);
    
    UpdateWheelSleep(wd);
    
    // Update visual wheel object
    
    if (needUpdateVisuals)
     UpdateTransform(wd);
    
    if (wd.grounded) groundedWheels++;
    
    // Calculate extended tire data
    
    if (computeExtendedTireData)
     ComputeExtendedTireData(wd, referenceDownforce);
   }
   
   // Apply aerodynamic properties
   
   float sqrVelocity = m_rigidbody.velocity.sqrMagnitude;
   
   Vector3 dragForce = -aeroDrag * sqrVelocity * m_rigidbody.velocity.normalized;
   Vector3 loadForce = -aeroDownforce * sqrVelocity * m_transform.up;
   Vector3 aeroAppPoint = m_transform.position + m_transform.forward * aeroAppPointOffset;
   
   m_rigidbody.AddForceAtPosition(dragForce, aeroAppPoint);
   if (groundedWheels > 0) m_rigidbody.AddForceAtPosition(loadForce, aeroAppPoint);
   
   // debugText = string.Format("AeroDrag: {0,6:0.} AeroForce: {1,6:0.}", dragForce.magnitude, loadForce.magnitude);
   
   // Handle impacts
   
   if (processContacts)
    HandleImpacts();
  }
...