Posi Traction

[TPISly-C10]

I have a GREAT running '84 305 with a 700R4, all original 57k truck getting about 12mpg avg, in town mileage. I would like to get a little more performance from the small block so I'm considering changing to a limited slip rear end with 3.42 gears. If I have a 3.08 and P225/75 R15 tires, I would only be adding 283 rpm in 3rd and 198 in OD (if I calculated right). My build sheet shows a "standard" real axle, code GQ1.
How can I determine the current gear ratio, size (8.5 or 8.6) and number of axle splines (28 or 30)? It's definitely a 10 bolt R/E, easy to count. Next, which is better, the: Eaton Posi Performance 19557-010; Eaton Truetrac 912A556; or USA Standard Gear Yukon Dura Grip 26015? I have budgeted $1,000.00 for parts. Any caution or helpful hints would be appreciated and do you think it would be worth it?. Thanks guys.

got yukon gear posi unit and brand new Yukon gear 3.42 just love it! 30" tire 4l80e/6L LS set up

 

[89GMCJimmy]

If you can find a 3.73 with a G80 that would be your best bet. Your stock 85' 305ci <200hp would benefit. Check the wrecking yard or 4x4 shops many Chevy/GMC owners going with 35"+ tires will swap them out. Lots of trucks and Blazer/Jimmy and 1/2 ton Suburbans had them. 1980 - 1988 had 28 spline. The Pre 1980 had 12 bolt and 89' and later 30 spline 10 bolt. Eaton true trac is a good option too.
The G80 is a differential designed by Eaton to work as a low-speed traction aid, it has full lockup during its operation. When in normal conditions, the unit acts as an open differential, letting the rear tires spin independently of each other but when one tire spins 120 rpm faster than the other, the Eaton locks up.

 

[89GMCJimmy]

You will not gain HP but the gearing will send you off the line much quicker and have 2 wheel traction not 1 slipping.
At 70mph with OD you should be around 2150 rpm

 

[Rusty Nail]

That's a great point.

Positraction certainly doesn't add any horsepower "off the line" or at 40mph, much less at highway speeds - unlike oh say, cylinder heads orrrrr new exhaust and headers, for example.

 

[Craig Nedrow]

I really Yukon gear, use it and no problems behind the 454. Here is the GM section....https://www.yukongear.com/shop-by-diff?model=GM
Go here, about page 3 for my locker swap: https://www.gmsquarebody.com/threads/1973-k20-350-to-454-swap.29840/
I like the 3.73 gears in my K10.

 

[89GMCJimmy]

That's a great point.

Positraction certainly doesn't add any horsepower "off the line" or at 40mph, much less at highway speeds - unlike oh say, cylinder heads orrrrr new exhaust and headers, for example.

Yes my comment that was poorly written was in regards to changing the gears to 3.73 would give him quicker off the line performance without any HP gains. I agree completely (heads, exhaust and headers) would help.
I have an 89' stock 350CI TBI begging to have 200hp, when i changed the stock 3.73 gears to 4.56 it felt like a new truck.
I run 285/75/16 at 75mph rpm's are close to 2500 in od.

 

[89GMCJimmy]

I have a GREAT running '84 305 with a 700R4, all original 57k truck getting about 12mpg avg, in town mileage. I would like to get a little more performance from the small block so I'm considering changing to a limited slip rear end with 3.42 gears. If I have a 3.08 and P225/75 R15 tires, I would only be adding 283 rpm in 3rd and 198 in OD (if I calculated right). My build sheet shows a "standard" real axle, code GQ1.
How can I determine the current gear ratio, size (8.5 or 8.6) and number of axle splines (28 or 30)? It's definitely a 10 bolt R/E, easy to count. Next, which is better, the: Eaton Posi Performance 19557-010; Eaton Truetrac 912A556; or USA Standard Gear Yukon Dura Grip 26015? I have budgeted $1,000.00 for parts. Any caution or helpful hints would be appreciated and do you think it would be worth it?. Thanks guys.

If you are certain you have 3.08 gears you can use this easy calculator.

gear ratio rpm calculator / converter

 

[Rusty Nail]

He's hot to change the gears, must have a bird in his ear. There is a poll.
Im gonna unwatch since he wants to prove me wrong.

Good luck new guy.

 

[SquareRoot]

Fwiw I broke my spider gears in my stock rear end 2.73 8.5 10 bolt. At the time I had a th350 behind the 454. So I put in this one.

GM Chevy 8.5" Chevy 10-Bolt Rearend TrueTrac - 28 Spline, Gear, Bearing Kit Package - 3.73 Ratio https://a.co/d/gqeXroX

It's gone up over 100 bucks since then. Thanks JB!

It still breaks them loose but. That's with a bbc. Now I've switched over to a 700r4 because I wanted overdrive to lower the rpms a bit. I'll probably get it back next week. My mechanic said it made a big difference in the rpms in od.

So like was said earlier I think the 3.73 one above would work well for you with that transmission and your taller tires. Mine are 275/60/15s. You still be under 1k too.

A 700 behind a 454? Subscribed.

 

[Redfish]

He's hot to change the gears, must have a bird in his ear. There is a poll.
Im gonna unwatch since he wants to prove me wrong.

Good luck new guy.

I don't think that was his intent at all. The man has 2.73 gears and is spinning one tire because he doesn't have any positive traction or limited slip. He is doing one of several possible upgrades. I personally would not want 2.73s and no posi-trac in my own truck. And it is HIS truck after all.

 

[texasmike]

You know, I thought I had made up my mind to make the switch but after driving the truck the other day, I’m still on the fence. It’s running so damn good, acceleration is a little bit better than what one would expect with an SB, high gears and smog emissions equipment removed. Passing gear works well and all runs smooth with no vibration or noise. In town mpg is around 13. Amazon is free shipping today (not a prime member) so I don’t have much time left, this time around, to decide???? And the price will only go up.

 

[DoubleDingo]

If it ain't broke, don't break it to fix it

 

[Turbo4whl]

If it ain't broke, don't break it to fix it

Or, you can't fix if it ain't broke.

@texasmike

Running smooth..... Drive your truck and enjoy it!

 

[SquareRoot]

If it works take it apart and find out why. While it's apart you have the perfect reason to upgrade.

 

[Octane]

My k10 fulltime 4x4 with a 350 and th350, with 3:73 gears gets me 12mpg,city/hiway combo.I'm thinking I'm doing good...but it's usually not doing over 55mph tho. So that helps. 2:73 or 3:08 gearing on his truck... good grief. With a 305 it sounds like a dog. Lol. I believe I'd change out that 305 for a 350. Or just put aggressive tread tires on the backend for better gription.