Author Topic: Need help choosing a new rear end ratio  (Read 2763 times)

Offline Joe-NC

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Need help choosing a new rear end ratio
« on: November 30, 2019, 05:44:14 AM »
I have an 8.5, 10-bolt rear end. It currently has a 2.56 ratio with a chewed up ring gear. Which is great because I couldn't stand this dog much longer. I have a TH350C tranny with a mild shift kit and no stall in the converter. The engine is an old 305 which I will swap at some point. I'm running 30" tires now and will probably never run bigger than 32". I hardly ever tow anything. If I do it would be a lawnmower trailer. I use it for work about 4-8 times a month which is 20 miles away. Occasionally I get on the interstate where I would be running 70-80 mph. I will never be taking long trips or off-roading. I want to go from 0-60 as fast as possible and still be able to run 70 mph for 30 minutes without the engine screaming too bad. Fuel efficiency is not a concern at all.

I've looked at the rpm charts, but I don't have a tach so I'm not really sure what range my engine likes. But as I said it will be swapped at some point. Maybe a 350 or possibly an LS with a 700R4.

So what gear ratio would you recommend for my rear end rebuild?

Offline 75gmck25

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Re: Need help choosing a new rear end ratio
« Reply #1 on: November 30, 2019, 06:06:21 AM »
My K25 has 4.10 rear gears, TH350, and 31" tires, so it runs about 2800 RPM at 60 MPH.  It puts you in a good power band, but sounds awfully "busy" in an old truck with almost no sound insulation.  However, this was how it was set up from the factory and it works well when the truck is loaded or towing.

I would think you may want to go with about 3.73 or 3.42 gears, which are common in squarebody trucks.  However, you have to decide how much RPM is too much.


Offline ehjorten

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Re: Need help choosing a new rear end ratio
« Reply #2 on: December 02, 2019, 09:08:23 AM »
I wouldn't go any higher numerically than 3.73:1 for your case.  I have a '77 K20 with 4:10:1 gears, no overdrive and 31" tall tires.  I can get on the interstate and go 70 MPH, but she is working hard and I can push it to about 80 for a short stint, but 70 is pushing it for cruising for a length of time.  I wish it had lower gears sometimes because that high RPM running coupled with the noise is fatiguing on the driver.  At 70 MPH it is turning a little over 3,100 RPM.  At 80 she is turning about 3,600 RPM.

With the same 3.73:1 gears it would drop it about 350 RPMs to: 2,850 at 70 and 3,250 at 80.

With 2.56:1 gears I would be turning about under 2,000 at 70 MPH, so bigger than a 1,000 RPM drop!  Run your truck on the highway at 70 in 2nd gear and you will get the feel for what it is like!  With 30" tires, 2.56:1 gears you will be turning about almost 3,100 RPM at 70 MPH in 2nd gear.
« Last Edit: December 02, 2019, 09:10:04 AM by ehjorten »
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Re: Need help choosing a new rear end ratio
« Reply #3 on: December 02, 2019, 09:25:11 PM »
Disregard if you have found your answer.
With the case you have 3.08 ring gear is all that will fit. next step is change out the case and RG P.
85 C1500 GMC SIERRA-donor

Offline Joe-NC

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Re: Need help choosing a new rear end ratio
« Reply #4 on: December 07, 2019, 11:16:43 PM »
Thanks for all the input guys. I went from 2.56 to 3.42. My case was no problem at all (8.5 10-bolt). But I did have change the carrier from a series 2 to a series 3, which added about $100 more to the cost. The total was $1,100. I'm sure I could have gotten it done much cheaper, but I had it done by East Coast Gear Supply in Raleigh NC. They only do rear end work and they came highly recommended from several local mechanics. They also stand behind their work with a warranty. I think my RPM on the interstate is just about right. Just like you said, I think 3.73 would be the upper limit for my setup. Maybe that's what I should have gone with, but the 3.42 was a good compromise. I'm still trying to get used to it, waiting for it to shift into 3rd, then realizing... oh yeah, I'm already in 3rd, ha ha. But it's so much better than that 2.56, which I didn't even know was supposed to be in a truck.

What puzzles and irritates me is that 4 mechanics over the past 10 years had no idea what the noise was coming from my truck. And I didn't have enough experience with rear-end noise to know what I was hearing. But now I know, and for anyone like me who's never experienced the sound of a bad rear end... it's a whining noise that goes away when you let off the gas. Maybe one of you guys with more experience will correct me if I'm wrong, but there are four adjustments that can get sloppy with age and wear. 1) the pinion depth 2) the backlash 3) the pinion bearing pre-load and 4) carrier bearing pre-load.

Can (and should) any of these things be adjusted (shimmed, etc) during the life of the rear-end to compensate for bearing wear? Or is it better to leave everything alone until it's time to replace the bearings?

Offline zieg85

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Re: Need help choosing a new rear end ratio
« Reply #5 on: December 08, 2019, 06:56:24 AM »
What usually happens is a pinion seals start to leak and someone figures it is easy to change the seal, which it is but you have to put the nut back on the pinion exactly where it was.  The ones I've seen are tightened too much, further crushing the crush sleeve which pulls the pinion gear away from the ring gear.
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Offline bd

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Re: Need help choosing a new rear end ratio
« Reply #6 on: December 08, 2019, 12:02:33 PM »
...It [is] better to leave everything alone until it's time to replace the bearings...

...or the gear set or address drive axle noise.
It's difficult to know just how much you don't know until you know it.
In other words... if people learn by making mistakes, by now I should know just about everything!!!
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Offline JohnnyPopper

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Re: Need help choosing a new rear end ratio
« Reply #7 on: December 10, 2019, 08:40:38 PM »
Well, first off, glad you're happy now!

Zeig, interesting take on the seal dilemma! Unintended consequences for the uninitiated!

I personally have not been initiated in the finer arts of rear ends (read=third member) of trucks. It has been my goal, so thanks for the details!
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Offline usmc

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Re: Need help choosing a new rear end ratio
« Reply #8 on: February 13, 2020, 07:39:48 AM »
Rear end --hey guys new here. I be working on 84 chevy short box two wheel, 350 ci with art shp heads, rpm air gap manifold, 650 carb, headers,rebuilt th350. What I need is a 8.5-8.6 10 bolt ,I would take a 12 bolt rear end. 3.08-3.42 locker. Where might a guy in mi look to find one . Thanks ia