models with better gear ratios
 

I'd like to eventually change the rear end on my 86 to a better gear ratio. I don't care if it's posi but that's a bonus for sure. I'm just wanting something around 3.23 to 3.50 so it works better with an overdrive transmission and is a direct swap. I have a listing of all the codes that the rear ends were stamped with at the factory. However, is there any particular year range or models that typically had the better rear end ratios, or do I just have to check every truck I come across? Would prefer to stay with the 10 bolt to save money. I don't need a 12 bolt.

Thanks

Re: models with better gear ratios
 

What gear do you have now?

I assume you already have an overdrive trans.

Re: models with better gear ratios
 

2.8x or something like that. Crappy is what it is.

Thanks

Re: models with better gear ratios
 

Likely a 2.73 gear.
Works great at 100 mph.

A 3.42 gear would be better.
I can’t recommend what year to look for.

Have you priced out a swap to a better gear in the rearend you have.

Re: models with better gear ratios
 

Not really. I was hoping to just swap the whole rear end as that takes less time and allows me to clean up the new rear end and get it ready in general while I still drive the truck with the current rear end.

Re: models with better gear ratios
 

I’ve done the same.
But that was when used stuff was only 10 years old. Not close to 40 years old now. A lot of stuff could be worn out by now.
Problem is it’s hard to judge condition of a rear end. They usually don’t make noise or leak until you use them.
You could end up spending a bunch more money and time.
That said I’d suspect the earlier models from before overdrive trans were used would be the rear to get. A lot of them had 3.40 or 3.73 gears.
The 80 gmc I bought new had 3.40 gear in it.

Apoligies if I’m getting you off in the wrong direction.

Re: models with better gear ratios
 

2nd on the earlier trucks having 3.42(3.40) as a standard. That is what my 75 has. It is a 12 bolt.

I would not trust any rear end stamps to be correct unless you know the vehicle's history. Very possible a PO replaced that 3.42 with a 4.10 in the past for better towing or a 3.08 while chasing better gas mileage.

I recommend you play with one of the on line RPM calculators and enter both 3.42 and 3.73 rear ends and your planned tire diameter and see what you wind up with.

Re: models with better gear ratios
 

Rebuild with what you have. Swapping out the entire may put you into rebuilding it anyways

Re: models with better gear ratios
 

From the "for what it's worth" department, I wouldn't go to any great expense to make the swap from 2.73 to 3.42
It's a lot of work to swap a very used rear axle for a minimal gain. You will barely see any difference. Even if you can get a "good" one, it is in the range of 35 years old. How good is it? The time cost of going through the brakes and unknown bearings?
IMHO, the money would be better spent on new gears for the one you already have.....unless, of course it has issues too?
Also, make the jump to 3.73

Only the last model year of this group of trucks was fuel injected. Carburetors do not do so well with cruising RPM (at speed) too low, so everything suffers. Performance and fuel economy are both poor.
Some of these trucks were geared as high as 2.56. Those things will roll down the road at highway speeds in the range of 1400-1500 RPM. No carburetor can cope with that. It is trying to cruise somewhere between the idle circuit and transfer to full jetting. I really don't get where the engineer's heads were on this?

Re: models with better gear ratios
 

All good points. I don't mind going through a rear before swapping it into the truck. I can do that in my spare time while I drive the truck in its current state. I'd only want to spend a couple hundred on the new rear anyway.


Eventually, I plan to do an LS swap. That's why I want a better rear. I've had an 01 Sierra with a 3.73 rear gear and it was great. 3.42s also work great behind LS engines with auto transmissions. 2.73s and 3.08s are meh at best.

Thanks

Re: models with better gear ratios
 

If you're looking for a different rear axle, don't depend on the RPO codes on the SPID sticker for that. There's too much chance that the axles have been swapped out in a truck's lifetime. The RPO codes could be the first thing you check, but always confirm the actual ratio itself.

Before you buy any replacement axle, do a quick check to see if it has the gears you want.

Put the truck in neutral, and jack up a single rear wheel. Attempt to spin that wheel. If the wheel spins easily, it has an open differential or a gov-lock differential. If the wheel doesn't spin easily, try jacking up the other wheel as well. If both wheels spin easily together, then it's probably a limited-slip differential of some sort.

To check the axle ratio of a open diff -- Rotate a single wheel twice, and the number of driveshaft revolutions is your ratio. The single wheel must be rotated twice to accommodate for the differential action.

To check the axle ratio on a limited-slip diff -- Rotate both wheels together for a single rotation, and the number of driveshaft revolutions is your ratio.

To be more certain, rotate the wheel(s) by 20 or 10 turns respectively, and divide the driveshaft revolutions by 10 to get the ratio.

Re: models with better gear ratios
 

Whether you get an axle to swap in or plan to re-gear the one you have, I would plan on going through it and changing all the bearings/brakes/seals. With as old as these trucks are getting its worth fixing it once and not having to deal with it again for another 20-40 years.

I re-geared my K5 from 4.56 to 3.73 with the axles in the truck. Took me 2 weekends, but I also had to rebuild both air lockers and that was a hassle. If I were you, I'd go on this calculator and put in all your specs and figure out what ratio you need to get the performance you are looking for, buy a gear and bearing kit and rebuild the axle in the truck. Torquing the crush sleeve is a pain but its definitely doable in a weekend. It would probably take as long to find and pick up another housing as it would to regear the one you have honestly.

https://spicerparts.com/calculators/...rpm-calculator


I sold a 3.08 10 bolt in SoCal for $50 a couple months ago. Housings are still out there, but are getting harder and harder to find and they are really heavy to move around. If the extra cost and work are worth it to you to not take the truck our of service for a weekend, buying a second housing may be the way to go...

Re: models with better gear ratios
 

that would be a plan also to save down time

Re: models with better gear ratios
 

I agree with the previous answers -

Specifically:

a) Rear axle ratios were pretty free expression from the factory, so any "good" ratio could be found in any truck

b) I would recommend rebuilding, because there's no guarantee what's inside the pumpkin of a forty year old truck, regardless of what it came with.

c) You don't say what engine you have, but for a nice running daily driven small block I have historically gone with a 3.42 for no overdrive, 3.73 with overdrive with a P235 ish sized tire.


Gives you a nice balance of off the line "pep" and highway economy.

K

Re: models with better gear ratios
 

I have 3.23 gears in the 85 K20. I'm thinking of going to 3.73 because the trans hunts even at highway speeds. I never use the overdrive it stays in drove all the time .

Re: models with better gear ratios
 

Thanks for all the replies everyone. I appreciate the info. We'll see where I go from here. I'm not in a hurry.

Thank you