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

 

[ekajkrats]

Welcome Chris. I am sure you will hear from some others on the board who are a lot more knowledgeable than me on axles.

In the meantime is a link that could be real helpful in you identifying your axles: http://coloradok5.com/axleguide.shtml

Also, when you get to 5 posts, post up some pics of your rig...you'll see or hear a lot from others of how much we love pics.

I have the opposite problem of you, and would like to move my gear ratio away from 3.07 (at least after I could get an overdrive transmission).

Another way to get your RPMs down is to up your tire size.

-Jacob

 

[tRustyK5]

Sounds like it has 5.13's to me. 10 bolt cover on the front doesn't help us much for ID...D44, 10 bolt(duh) and Dana 60 fronts all have 10 bolts covers...

Lower gears (numerically) will get the R's down, but performance will suffer. I'd consider an OD swap like the 700R-4 to get the best of both worlds.

Anyways, to verify ratio pull the covers off, and you'll find ratio stamped on the edge of the ring gear. Prolly need a clean/service anyways.

What tach do you have and is it reading right? Lots of aftermarket tachs have 4-6-8 cyl capability, and if set on the wrong one will give you an incorrect reading.

 

[Greg72]

Welcome!

Couple quick comments to get you started.....

1. Pull the rear diff cover and read the markings off the ring gear (or take a photo and post them up). The stampings will reveal the tooth count on both the ring & pinion gears and from that you can calculate the gear ratio accurately. As stated, your tach might be off so you can't trust that for figuring out gearing.

2. Trying to optimize highway RPMs without an overdrive transmission is going to make your truck really doggy in around-town driving. By the time you get the highway RPMs nice and low you've given up all the torque multiplication that a nice deep set of axle gears provides. Driving a truck like ours with 3.07 or 3.73 gears isnt much fun, except for steady state highway cruising.

-G

 

[72blazerguy]

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?

I have read a 1,000 posts of people asking if they should swap axles for 3/4 ton. Not usually the other way around

I run 4.56 gears w/ 35" tires and TH350 tranny. The blazer runs great, but at high rpm above 65 mph. I had the TH350 rebuilt and wished that I had not spent the money on that tranny and just swapped in a 700R4. It's got a lower 1st gear and the overdrive.

I would not go with higher gears in the differentials, but look into different transmissions. As mentioned above, higher gears will make your rig seem more sluggish off the line. Also, you will want the lower gears if you are going to 4wheel at all. The 205 tcase only has a 2:1 reduction, so you will want to stay as low as possible in the diffs.

 

[ChrisCostlow]

Thanks for the responses!

So it's looking like I'll keep the 3/4 ton axles either way. I've been reading about the 700r4 swap and it seems like that's a high-dollar way to go, what with the adapters and such. BTW, how does one use a tranny that has OD with the stock steering column?

I think I can wangle about $2500-3000 out of my Wife over the summer... would that be enough do to have someone swap the tranny for a 700r4? Or should I maybe go with the 373 gears and get some of the rust taken care of to boot?

I knew I came to the right place!

 

[K85 Octane]

You need to go to this and ask whoever shows up.

http://coloradok5.com/forums/showthread.php?t=307285

Since you are already right there, it wouldn't be hard and I'm sure some of the guys wouldn't mind BSing about your rig. Personally, if you haven't been driving it long, I wouldn't do the trans yet. Keep driving the truck and wait for something to break and replace. If you are thinking trans, there will be driveshaft modifications needed, gears to take advantage of the trans, adapters, and TV cable stuff. Keep an eye out for a late 80s K case 700r4 while you work on the truck.

If you ever make it out of the mountain my way, PM me. Also, post a few more times, even about nothing to get your post count above 5 or 10 (can't remember) and show us some pictures

 

[bp71k5]

One more possible cause of your gear ratio problem:

My first trip to Moab, I packed up my wife and my friend and gear and we headed out onto the start of the 1000 mile trip. As we we pulled onto the freeway, I noticed the motor was revving really high and I wasn't moving that fast.

My wife points to the transfer case shifter and asks if its "supposed to be in that spot"? Oh sure, that's the way it always is. So I keep driving and eventually decide something is wrong with the transmission. We lookup a transmission shop and make an appointment to have them look at it. After waiting for three hours, the verdict was that nothing is wrong and maybe I just need different axle gears or the shift points were not adjusted right.

So we start heading back home because there's no way it would go 1000 miles like that. Halfway home, I checked the transfer case shift again and found out I'd been driving in 4 low the entire time.

So by the time we got to Moab, it was late at night in the middle of the biggest rainstorm Moab had seen in years.

Moral of the story, check the basics first!!!

 

[RckyMntnKng]

Just another 2 cents -

A transfer case doubler solved alot for me...

I had 4.11's running 40" tires which was fine on the highway at high rpm, not so much in town. (stock 205 t-case)

The doubler can let you run the second 203 lower t-case gears in 2wd so you get that "2-lo" spot thats just about right for the standard stop n go driving on bigger tires.

It worked for my gearing and tires but would have to research what your ratios would end up at.

 

[LVJimmy]

Welcome!

Couple quick comments to get you started.....

1. Pull the rear diff cover and read the markings off the ring gear (or take a photo and post them up). The stampings will reveal the tooth count on both the ring & pinion gears and from that you can calculate the gear ratio accurately. As stated, your tach might be off so you can't trust that for figuring out gearing.

2. Trying to optimize highway RPMs without an overdrive transmission is going to make your truck really doggy in around-town driving. By the time you get the highway RPMs nice and low you've given up all the torque multiplication that a nice deep set of axle gears provides. Driving a truck like ours with 3.07 or 3.73 gears isnt much fun, except for steady state highway cruising.

-G

I'm very happy with my 3.73s and 33" tires. They perform well on the street and wonderfully in 4 low.

As said earlier check the basics. Tach, actual speed, gear ratio xfer case, tc slipping.

 

[MrSchaeferPants]

Whats the plans for the truck? Just a cool fun DD? You have what most people want, 3/4 ton axles with low gearing. I think it's a smart move keeping the axles you have. Do you like the 33s or would you even prefer some beefier bigger tires? This would slightly lower rpms.

With the high rpms, what are your concerns? Just that the engine is running at a high rpm, or trying to get better MPGs, or a bit of both perhaps? There's no easy answer, multiple ways of lowering your rpms, but what you do should be dictated by what you want to achieve with your truck.

A trans swap to a 700r4 isn't terribly hard; more pricey than difficult. But you need everything, what carb is on this engine, you'll need an adapter for the TV cable, drive shaft length & crossmember coming from a th350, controlling torque converter lock up, new trans cooler lines, and also trans cooling is now a much bigger concern. Also, a 700r4 is not something I would buy used like a 350 or 400. Like I say in all OD swap threads, doing it souley for MPG is fruitless. The money in MPG saved swapping to an OD trans will take you years to break even.

Swapping gears could also be pricey if you took it to a shop, but definitely a cheaper option than swapping a trans. You would still have some decent power with 3.73, and it would get you near your goal of 2500. If it's nothing but a DD, I'd go this route. If you do go with 1/2 tons with 3.73s or similar, its an easy cheap swap. Blasphemous, but cheap and easy In that case please stop by, I'll swap you my 1/2 tons with 3.08s, and do all the work for free, and buy YOU the beer

My opinion, keep your power train as is for now if it all works just fine. Put that money towards those other things like rust and topper. A 6 mile commute help you out a lot Our old rigs were designed to be beasts, not smooth running commuters.