Posi Traction

[texasmike]

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.

 

[Rusty Nail]

The short answer...would it be worth it? No.
Positraction adds exactly zero horsepower, it makes both rear tires spin instead of one.

A 1984 305 has very weak cylinder heads. My opinion for $1,000 is to source a pair of *perimeter bolt #601s and have them built at a machine shop, modifying with larger diameter exhaust valves, a three angle valve job , "Z-28" valvesprings and re-install using copper shim head gaskets.

You'll have money left over.

Want to spend more money?
Buy a pair of FLOWTECH brand 1.5" primary headers and build a dual exhaust system using whatever brand of muffler you like (Dynomax Super Turbo is bang for the buck) and make certain to connect the two sides with a "H pipe" or balance tube behind the transmission.

All in? Less than $1500 if you don't spend a dime making it look cool. Er. Cool-er.

 

[SquareRoot]

Nitrous for when you get the itch. After you scratch it, you can go right back to the lowly, reliable 305.

 

[Redfish]

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.

That's the question, IF you currently run 3.08 gears. Dropping to 3.42 will give you a little better acceleration and probably won't hurt your mileage very much. Unless it is raining or you have already done some work to your 305, you aren't going to be spinning your tires much anyway so "limited slip" won't help a lot. Obviously positive traction isn't adding any performance unless you have enough horsepower to easily spin the tires already.

You can feel the difference in acceleration between the 3.08 and 3.42 gears. It still won't be fast though.

 

[75gmck25]

I’m not sure that changing axle ratio and going to a slip rear end is the best performance upgrade to start with, and the minor difference with a 3.42 is definitely not worth the trouble. Since you have an overdrive transmission, if you go that route I would use a 3.73 or even a 4.10 rear end. It will make little difference in your in-town gas mileage since you never really get the rpm up anyway.

Duels exhaust/headers, better intake and carburetor, more aggressive cam and increased timing are more typical first improvements for a relatively stock engine.

 

[texasmike]

Thanks for your comments everyone. It’s a long bed and I’m always breaking the right real tire loose because there’s no weight in it. I thought the posi trac would help that issue and while I had it apart, for a couple of hundred $ more, I might as well change the gears.

 

[scrap--metal]

It’s a long bed and I’m always breaking the right real tire loose because there’s no weight in it. I thought the posi trac would help that issue and while I had it apart, for a couple of hundred $ more, I might as well change the gears.

Your thinking isn't far off...

I changed gears from 2.73 to 3.08 in a '76 Monte Carlo with a Target Master 305, and there was a noticeable change in acceleration. The car just drove better and I didn't lose any top end with the TH350 either. We also put a G80 locker in the 10 bolt rear end at the same time. I never blew it up, but I never beat on it too hard either. The locker was nice when things were wet or snowy. My project was completed for cheap with junk yard parts though.

If you have a $1000 to burn, I think it'd be worth it. Just don't expect it to feel like it has a new 350 in it. You need to be content with your existing 305. $1000 towards some new heads or a roller cam would make more of a performance difference, so that's what you should weigh your decision against.

How can I determine the current gear ratio, size (8.5 or 8.6) and number of axle splines (28 or 30)?

I'm pretty sure it should be 8.5" with 28 splines. 30 splines weren't used until later model trucks, and probably were not swapped into you 57K mile ride if it still has the 3.08 gears and an open differential.

For your application 3.73s are probably a better choice than 3.42, like @75gmck25 suggested.

 

[MikeB]

3.73 gears are typically considered to be a great fit for a 700R4. Even more so with a 305 and tall tires (compared to a car).

 

[Big Ray]

3.73 or 4.10...
Multiple those by .72 to calculate your effective o.d. ratio...

3.73 = 2.68
4.10 = 2.95
Some other options
4.56 = 3.28
4.88 = 3.51
I had an 87 with a 305, 2.73 gears without overdrive. Damn near got me killed so many times I lost count. I did a lot of traveling on 2 lane roads, passing was an adventure. Looking back the first thing I should have done was regear it. And thrown the 305 in a ditch lol!

BTW, 10 bolts were 28 spline up until mid 89 iirc.

 

[Bextreme04]

My 2011 Suburban has a 8.6" 10 bolt and came stock with posi and 3.08 gears. It has a 5.3/6L80 and we use it to tow our small camper for family trips. I just recently changed out the 3.08 gears for 4.10's and it made a world of difference. The 6L80 has a dual overdrive.. so the 6th gear is 0.67:1. It sits at about 2200 RPM at 75mph, which is just fine for an LS or SBC.

 

[texasmike]

Probably a dumb question but will changing the rear end gears affect the speedometer reading?

 

[Redfish]

Probably a dumb question but will changing the rear end gears affect the speedometer reading?

Yes

 

[DoubleDingo]

Yes

^^ This ^^

 

[82sbshortbed]

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.

 

[texasmike]

Thanks Doug, I'm considering the same setup as you. If you don't mind, I'd like to get back with you early next year to see how everything is working for you with the 700r4. I did the tire turn and driveshaft test and found out I have 2.73 gears so I would really see some difference. I'm getting about 12.5 mpg in town and still haven't taken any long trips to check hwy mileage. The 3.73 would add 475rpm at 40mph in D and at 70mph, +582rpm in OD. Not sure how the posi trac would affect mpg. I'm past the racing and who has the biggest dick phase and would just like to have a little more performance when I really need it, like passing some slow poke. I've finally got the small block and Q-jet timed and tuned to perfection and it starts and runs as great as the truck looks. The fellow with his hand on the truck is a long time friend, my employer for 27 years and the man who bought the truck in 1984 for the company fleet.

BTW, did you have the trans shop change the speedo gear?