How to identify if a transmission is double OD

Yeah. Pretty large gear jumps. I know a few old timers that did not like them after they had them done. They always said You have to be running way over legal speeds to make them worth while.

 

You should ask your transmission what it identifies as, to be safe. It may say it feels like a twin stick somedays, and a Rockwell 9 speed other days.

On a serious note, I've never seen a 15 speed with double over myself. Just read that it can be done in this thread.

 

Tags on old transmissions can lye as people have changed things internally.
If it is that 15 speed you should be able to be in low low behind reverse. Shift 1, 2, 3 and split to low. Then shift straight back up next to the gear by reverse. Then continue on with the pattern. Some other guys break the pattern a few gears higher. Either way I thank you only get 11 gears in progression.
So why is it a 15 speed? I guess all those other numbers were used up?
In high range if the splitter causes the truck to go faster when shifted it is an OD back box. That is the only way to have a double OD along with a OD front box.

 

That is Single overdrive trans. U shape patter is solely in top cover. That used to be standard, and H pattern is a optional. All in top cover.

 

There were some ######## along the way. Drove one in an old oil drilling rig on its way to Charleston port. It was going to Kuwate after the invasion. It had a round chrome knob like the old 13 speeds. In low it had 5 gears in progression, then in direct it had 5 more and in OD 5 more. A true 15 speed.

 

Those were rare. They used the 3 position 13 speed knob instead of the reduction switch on the dash. They also had 3 reverse gears. Friend of mine had a ‘86 Freightliner with one. It was in it when he bought it used in ‘91. It was built with a 13 but the original owner swapped it in. Trans was probably 15 years older then the truck at the time. Lol!
I drove it once and being used to a 13 and that knob it was hard to keep myself from using the button to split up.

 

Best way is find an old truck mechanic with grey hair and have him drive it and tell You what it is .