Snippet to calculate xSpeed depending on distance travelled and time given in milliseconds.

CustomSpeed.as

private static var FPS:Number = 25;
private var movingMc:MovieClip
private var stillMc:MovieClip
private var orignalXPos:Number = 0;

// Set a default value if setter isn't being called
private var xSpeed:Number = 2;

// Constructor
public function CustomSpeed():void
{
  orignalXPos = movingMc.x;
}

// Setter function
public function set _speed( timeMS:Number ):void
{
  var distance:Number = Math.abs( orignalXPos - stillMc.x );
  var time:Number = ( timeMS / 1000 ) * FPS;

  xSpeed = distance / time;
}