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mechnical overdrive

1719 Views 18 Replies 3 Participants Last post by  docwatson
I heard about a mechanical overdrive (four speed) automatic tranny made in the mid 90's which might work in my 1990 Cummins 5.9 250. It currently has a 3-speed automatic. I have also been told there is a mechanical overdrive tailshaft I could use on my current tranny. Do you folks know which model numbers I should be looking for?
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Thanks for your reply. it sounds like I can't use any available overdrive tranny without some adapting. Is it feasible to use an electric switch of some sort to engage the overdrive? I had a Triumph GT6 with a manual tranny and electric switch for the overdrive.
Thanks again,

Are you aware of any fitment problems? The truck has the Cummins adapter for the 3-speed. I suspect I will have to use another driveshaft or shorten the one I have. What about the torque converter and pilot shaft?
That is all good news.
Well guys,

I think I saw one on eBay for a lot less. I am still sorting all this out. I would have to drive a LOT of miles to pay for a $3000 overdrive. If I saved 10c a mile, I would have to drive 30,000 miles. Not a likely scenario for a 24 year old truck.
I have a 3.58 Dana 60 rear end from a 1999 2500 I plan to install under the truck. It is a converted airport tug on a 1990 Dodge 250 chassis. It is a "bobtruck". I will pull a trailer with it, but need a taller ratio for running empty. That is why I would like an overdrive. I am looking around for the 47rh you recommend. Not sure what you mean about the TC. Are you suggesting I not use the one from my 3-speed?
Wow, so many factors. I will look for a 47rh with locking TC. My truck has only 28,000 miles on it, so someone might want the 3-speed tranny and TC. I can drive a 5-speed, so I will consider that. I started driving in 1959 and bought my first automatic in 1988. Right now, I am looking for a simple conversion to get it on the road, and installing a clutch is just one more factor to slow the process.
I had a thought. Since I will be engaging the OD with a manual switch, could I use a transmission from a gas model, like a V10 or 360?
Guess I will have to upgrade my membership before I can see the classifides. What is CompD?
Like I said, just a thought. Gas models are more common, but I want it right or might as well stay with the 3-speed. I found CompD.
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