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Hello world....
I have a '47 chev 1/2 ton with the usual 235 engine and 3:55 rear end upgrads. I'd like to get better gas mileage and lower rpm's (at 55-60mph). I've seen references to overdrives in the forums but this seems all a bit exotic to me. Do you think this is a good thing to do? Where do I find an overdrive? etc. etc.
Looking forward to hearing from the A/D gurus.
Cheers
Les

Hey 47, I'm not an A.D. guy and definately not a guru but I can point you in the right direction. I've been looking for an overdrive for a couple of years. I'm not exactly a mechanic so I didn't really understand what they were, I just knew I liked the results. Now I know that it is an auxilary 2-speed transmission that will go somewhere between your transmission and the rear axle. I have one that I just bought recently from Gear Vendors. It bolts right on to my sm420. I will be able to overdrive 2nd, 3rd, and 4th. You can set it so that you can engage it manually or set it to shift for you electronically between 4th and 4th over or between 3rd and 3rd over. 4th over will give me the equivilant of 3:08 on the highway with my original 3:90 posi. Best yet, I get to keep my 4spd with granny and 3:90 rear which can be so useful for my region and activities. I see you're from B.C. so I imagine there are some hills where you live. I do have to have the driveline shortened and I have to have my rear torsion bar crossmember fabricated to allow clearance. The later probably won't be a problem for you unless you've upgraded to torsion bar IFS. Warning, the unit and the process is not cheap. www.gearvendors.com They can answer your questions or maybe can tell you where you can get one that will match your application. I hope this helps.

With 355 gears you should be about right. Maby some larger tires.

47 Chev--

Gear Vendors products are nice but typically expensive--arguably the only overdrive option for the SM 420.

For an AD truck certainly a cheaper and still effective approach are electronic overdrive transmissions made by Muncie and saginaw. Bolt up to your existing bellhousing--gotta jerry rig a shifter and linkage.

The saginaw is stouter and draws a higher price assuming that the seller knows what he has and the muncie is lighter weight and in my mind fully capable for an AD truck. These were made for GM and other model cars 55--64 or somewhere near that. Typically sell for $50 -- $150 at flea markets, parts sales and on ebay.

The electronics involve some effort but nothin spectacular although some bolters wire them up quite elaborately for gear splittin and the like--overdrive stepdowns are typically 30%--works well with a 3.55 rearend--at least it works fine on my 55.1.

Hello Buzz....thanks for the info. Actually, thanks to you all for your thoughts. Buzz...what part of the O/D is electronic? and if I went to a boneyard, how would I recognize on of these transmissions. Could I retain the torque tube or do you think that woulld all have to go as well?

47 chev, another option is a New Process A833. They were used in the 81-86 Chevy trucks. It's a 4 speed manual and fourth is an over drive. No electrics. It looks like a Muncie on steroids. If you find one, get the linkage with it. I'm collecting parts to replace the 465 in my 68.
Larry

on that A833, you hafta get the bell housing it came with - the NP was a Mopar tranny and GM made a special bell houseing to mate it to the small block - not sure it'll fit the 235, and no torque tube possibilities prolly - tis a good one tho, and I think I'll be using one behind a 283 in a 57 panel

Bill

The A833 I have is Chevy bolt pattern.Here's a thread on Inliners.

http://www.inliners.org/cgi-bin/ultimatebb.cgi?ubb=get_topic;f=2;t=000453

Larry

And if you've got the big bucks for a gear vendors, here's two more options.

Richmond truck 6 speed ($3000+)
1 1/8 shaft with 10 splines
4.01 1st gear
.73 Overdrive 6th gear
mid mount shifter
Fits where an SM465 went 68 and up if you shorten the driveshaft. You can replace the SM420 with a little machine work on the bellhousing.

Ranger torque splitte from Advance Adapters for $1500. 2 models to fit in front of any chevy trans about $1500. .73 overdrive Moves the shifter back 7 inches. I like these, others on this forum have complained of problems.

Neither of these work with a TT rear.

Hay 47... I just finished installing a OD transmission in my 53. To recognise a 3 speed over drive transmission just look for the solenoid sticking out of the left side of the transmission (as viewed from the rear) and the govener sticking out of the right side near the rear of the trans.
You will have to go to an open drive shaft and rear end. I used a 55-1 rear end and housing with a mid 60's 3.90 pumpkin. This geves me a final OD drive of about 2.73 on the highway. Works great.
It was a lot of work but well worth the effort.
Tom

I take it you want to stick with a manual trans.The 700R4 is a great OD trans.but it is automatic.I am running one behind a 350 V/8 in my 55,with 3:90 rear.It works great,cruises at 65 to 70 MPH,and gets 20 MPG.

There's always the NV4500...

I have a sm420 in my 55.1 with a 390 rear end. We are assembling parts from an old overdrive tranny, rear half, to make a divorced overdrive. Won't know until we get it all together tho. Not much money in it. - Scott