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I just bought a 55-59 3sp overdrive. I plan to put it behind the 235 in my 31 pickup. I need to get up to speed on this tranny. What are the gear ratios? The rearend has 3:42 gears. It has a small lever thatI assume uses a push/pull cable to operate the overdrive. What is the electic solenoid for and how is it wired up. Thanks

There is lots of information written in this forum about OD's.
My recommendation would be to pick up a 55-59 shop manual which has all the information needed, wiring diagram, trouble shooting, etc. GM used OD's from 55-69 in cars and trucks.
In addition to the solenoid you should have a govenor (mounted on the rear housing) pull cable, kick down switch, relay, and wire loom.
Fred

Although an OD came from the factory with a kick-down switch and a relay, you can hook one up with just the cable and a toggle switch.

This website has l-inks to a bunch of OD documents. I just put one in my burb and this helped a bunch.

The front part of the transmission is just like a normal Chev 3 speed. The back part is a planetary unit with .70 overdrive built by Borg Warner. This is a pretty good over view of the OD http://www.hydratech.com/pctc/Public_Documents/bwwiring.pdf I would not put an OD in front of 3.42 gears. They are a lot more driveable with 3.90 or 4.11. I have OD in all my Y-blocks.. have for almost 50 years. I have an OD that I am going to put in in a GMC, but it will be with 3.90. I bid on an OD on eBay last week. You may be the one that outbid me at the last 5 seconds, but there will be more.

Is the OAL of the standard 3 speed and the OD unit the same so it can be put in a truck with the torque tube?

As far as I know, there was no OD trans built that will accept the torque tube set up. They require the slip joint of the open driveshaft. I swapped to a '55 first rear end.

Thanks Cletis, that is what I thought. This OD question comes up quite often and I think a lot of folks with pre 55-1 think all they have to do is swap out the tranny. Obviously a lot more to it. But you know how so many people ask a question but don't say what truck it pertains to and get the wrong impression.

Thanks to everybody who took time to reply. I paid $75 for the tranny at a swap meet. Straight 3spd transmissions were going for about $50. It's always fun learning new things.

You made a terrific deal @ $75 IMO.

Kenneth's advice on using 3.90 or 4.11 gears is right on. I'm using the 3.90s and that makes a 2.73 final drive. It's slows the 248 down on hills at highway speed. I don't think it could handle any higher ratio.