Need some Differential/Gear Ratio/Drivetrain help.

[ChrisCostlow]

Hey Fellow Travelers!

I bought a 72 CST a while back and am finally in a position to be able to spend some money on it. While I can turn a wrench just fine, there are tech things I don't know about the drivetrain. I'd appreciate any help and advice you can offer.

It's got 3/4 ton axles. The front diff cover has 10 bolts and the rear has 14. I can't find any identifying marks but haven't gone so far as to take a wire brush to 'em or anything. Should I keep the 3/4 ton hardware? It's not going offroad, but I live in the mountains so it'll be spending plenty of time in 4x.

It has a moderately strong feeling 350 with a TH350 tranny. I think the tranny might need to be rebuilt because of the way it shifts. 2nd at about 20mph and 3rd at about 35mph. The shifts are very positive though.

My biggest complaint is that it's turning 2500 rpm at 45 mph with 33" (305) tires. The gear ratio calculator says that's a 5.45. Can that be right? That seems awful high to me. The calculator also says if I want to go 70 at 2500 I'll need somewhere around 3.50. As it stands, about all it's good for is the drive down the boulevard to work and back, about 6 miles, and it's turning about 2800 at 50mph. I drive it daily.

On the plus side, she's got some rust, but at least I can't see the ground from inside! The dash has never been cut. The motor shows a little blue smoke at startup but doesn't smoke going down the road, at least not so's I can tell by looking in the mirror. The single-wall top is going to need work and I'm neither a body or fiberglass man, so I'll have to farm that out I think.

So:
Keep the axles or look for 1/2 ton stuff?
How to identify the axles I have?
Is 70 mph at 2500 rpm feasible?
If I keep the 3/4 ton axles, what's it going to take to get the gear ratio down?

That's all I can think of for now... You know I'll be back though LOL.

Thanks in advance for any help/advice.
Chris

[ChrisCostlow]

No help?

[Bigdav160]

Keep the axles or look for 1/2 ton stuff?

I'm not sure why you would want to do that. That would force you to buy new tire/rims also

How to identify the axles I have?

Plenty of axle identifiers on the web. It's likely you have a Dana 44 in the front and a GM "corporate" axle in the rear

Is 70 mph at 2500 rpm feasible?

Sure. Gears and (possibly) differential cases

If I keep the 3/4 ton axles, what's it going to take to get the gear ratio down?

See answer above. Some might suggest an overdrive unit like gear vendors but I would prefer the gear set swap.