Thats a good point about the 5 forward gears.
When you go into OD, its your fourth gear but after that, you get lock up. After OD at a certain rev point and if the engine temp is right, you get torque converter lock up. Try it. When you accel in an auto, the rev counter always rises before anything happens. When in OD at about 60, gently press the pedal so as not to kickdown and watch the rev counter move in unison with the car speed.
I believe this is temperature controlled and wont work from cold.