Transmission swap ideas

[72jenny]

I bought a '72 with a 350 engine that has a mild cam and is bored to .040. However, my truck has a bad tranny and will only do about 45mph without going to really high RPM. So I was looking to do a tranny swap on it and needed some ideas.

I'm not sure what tranny is in it at the moment, just know it is a 3-speed. I looked at the numbers on it and saw "GM20" as well as "3925647". A quick google search showed this to be a low performance Saginaw 3-speed for a 68-69 Camaro.

Is there any ideas of what transmission I could swap in for this? This is a budget build so I will be cruising craigslist for a cheap secondhand tranny.

[rocks409]

Sounds more like a clutch problem than trans.

[Mike Bradbury]

No clutch in an automatic, could be a variety of problems that could be easy or expensive to fix. Best to get it diagnosed by a reputable shop before you go chasing your tail on this one.
Your tranny is probably a Turbo 350 or 400. on mine I was up around 3300 rpm at 65 MPH.
What rear gear do you have? I had agh 400 with a 4:11 rear gear. Could be your torque converter is shot.

[72jenny]

Originally Posted by rocks409

Sounds more like a clutch problem than trans.

Originally Posted by Mike Bradbury

No clutch in an automatic, could be a variety of problems that could be easy or expensive to fix. Best to get it diagnosed by a reputable shop before you go chasing your tail on this one.
Your tranny is probably a Turbo 350 or 400. on mine I was up around 3300 rpm at 65 MPH.
What rear gear do you have?

Sorry, forgot to add that it is actually a 3 on the tree. Is there a reason you would say clutch problem? Doesn't feel like it is slipping at all is the only reason why I ask that. I would likely replace the clutch as well during the job.

I bought the truck with the engine upgrades and tranny so I don't know what the rear gearing is. I believe it is just stock since I don't think the PO changed it at all. I bought the truck in non-working condition and knew about the tranny problem then. The PO just said the tranny wasn't able to handle the power from the engine.

I bought this truck as a project to learn more about this type of work so I prefer to do it myself rather than bring it to a shop. Got most of the problems on the truck sorted out just taking care of the tranny now and she should be ready for the road.

[vectorit]

Sounds to me as though your truck is set up with low gears in the axle.
If the axle is set up with gears like 4.57's, you are going to be high rev'ing at freeway speeds or even on the streets.

Tell us more about your truck.
C10, K10, C20, or K20?

Take a picture of the rear axle for us if you do not know what model it is.

[highwinger]

It might be worth checking what rear gears you have. The speed and tach are predictable knowing the gears and outer tire diameter. There's plenty of online references and apps.

I just bought my burb and checked my setup a couple ways. First, I used a phone app that showed my speed using the gps which makes sure the speedo is correct. Mine wasn't but it's a Digital Dakota, not stock. To check the gearing, I jacked up the rear, put it neutral, put a chalk mark on the drive shaft near one of the round parts of the u-joint. Put a chalk mark on the rear tire too and counting how many times the driveshaft rotates with one revolution of the tire. Mine rotated between 3.25 & 3.5 times, closer to 3.5, so I believe its a 3.42.