problems with ros loop iteration

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Hi, I am having problems with the amount of time this loop iterates.

 #include <ros/ros.h>
#include <geometry_msgs/Twist.h>


class GoStraight{
public:
  GoStraight();

private:
  void callback(const ros::TimerEvent&);
  ros::NodeHandle ph_, nh_;
  ros::Publisher vel_pub_;
  geometry_msgs::Twist vel;
  ros::Timer timer_;

};

GoStraight::GoStraight(){
   vel_pub_ = ph_.advertise<geometry_msgs::Twist>(
    "/cmd_vel_mux/input/navi", 1);
   vel.angular.z = 0.0;
   vel.linear.x = .2;
   timer_ = nh_.createTimer(ros::Duration(3.0), &GoStraight::callback );
}

void callback(const ros::TimerEvent& event){

   vel_pub_.publish(vel);

  ROS_INFO_STREAM("Sending random velocity command:"
      << " linear=" << vel.linear.x
      << " angular=" << vel.angular.z);  


}

int main(int argc, char** argv){

  ros::init(argc, argv, "g_straight");
  GoStraight g_straight;
  ros::spin();

}

    this is the output that I get:
    [ INFO] [1403283564.623071038]: Sending random velocity command: linear=0.2 angular=0 at time =0.07
    [ INFO] [1403283564.723195177]: Sending random velocity command: linear=0.2 angular=0 at time =0.07
    [ INFO] [1403283564.823198861]: Sending random velocity command: linear=0.2 angular=0 at time =0.07
    [ INFO] [140328

3564.923198642]: Sending random velocity command: linear=0.2 angular=0 at time =0.07 [ INFO] [1403283565.023199258]: Sending random velocity command: linear=0.2 angular=0 at time =0.08 [ INFO] [1403283565.123149431]: Sending random velocity command: linear=0.2 angular=0 at time =0.08 [ INFO] [1403283565.223199416]: Sending random velocity command: linear=0.2 angular=0 at time =0.08 [ INFO] [1403283565.323155826]: Sending random velocity command: linear=0.2 angular=0 at time =0.08 [ INFO] [1403283565.423198781]: Sending random velocity command: linear=0.2 angular=0 at time =0.08 [ INFO] [1403283565.523200109]: Sending random velocity command: linear=0.2 angular=0 at time =0.08 [ INFO] [1403283565.623197323]: Sending random velocity command: linear=0.2 angular=0 at time =0.08

I would like an output that goes like this:
[ INFO] [1403283565.523200109]: Sending random velocity command: linear=0.2 angular=0 at time =0.0
[ INFO] [1403283565.623197323]: Sending random velocity command: linear=0.2 angular=0 at time =1.0
[ INFO] [1403283565.523200109]: Sending random velocity command: linear=0.2 angular=0 at time =2.0
[ INFO] [1403283565.623197323]: Sending random velocity command: linear=0.2 angular=0 at time =3.0
[ INFO] [1403283565.523200109]: Sending random velocity command: linear=0.2 angular=0 at time =4.0
[ INFO] [1403283565.623197323]: Sending random velocity command: linear=0.0 angular=0 at time =5.0

I am having a hard time understand how the rate.sleep() affects the speed that the ROS loop iterates and help is greatly appreciated.