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Cowpatty69
Cochise

USA
226 Posts

Posted - 28 Nov 2017 :  9:18:24 PM  Show Profile  Reply with Quote
Maybe more information is needed but I thought I'd throw this question out there. We are in the process of replacing the carrier in our 10 bolt 8.5 from an open rear to a limited slip carrier out of a 77 TA. It has like 2.41 gears, give or take. My son Benjamin talked me into picking up a set of 3.73s at the Pontiac Nationals- price was right.
The 78 Bird has a stock Chevy 350 that came in it with a stock T350 tranny and stock 15" wheels running 70s all around. I'm thinking with 3.73s he may have the car sideways 24/7-which I'm sure he'd like until the car ends up behind the cop shop. Any idea how the 3.73s are on the highway? I'm wondering if 3.23s or 3.42s might be a better bet for frying the hides AND getting some highway mileage? Thoughts? Thank you!

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VA-23707
Tribal Scout

USA
4 Posts

Posted - 29 Nov 2017 :  08:24:34 AM  Show Profile  Reply with Quote
Here is a link to a calculator that will allow you to change the gear ratios and tire sizes, and see where the rpm and speeds are. Be aware that the first rpm-speed chart is in metric, with the good ol' Merican below it.

https://www.engineersedge.com/mechanics_machines/transmission-gear-ratio-calculator.htm

Rich

Rich Senn
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Cowpatty69
Cochise

USA
226 Posts

Posted - 29 Nov 2017 :  7:40:05 PM  Show Profile  Reply with Quote
Thanks Rich- I'll give that a try!
Patrick

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cortcomp
Coyote

USA
5417 Posts

Posted - 29 Nov 2017 :  7:43:15 PM  Show Profile  Visit cortcomp's Homepage  Reply with Quote
I find 3.23s can even be too much on the highway behind a TH400, but then again i'm doing 85 on the highway, and i prefer quiet cruising at low rpms (i may be a closet LS fan!?)

IMHO, without overdrive, 3.23-3.42 are about a perfect balance, depending on tire height. With OD, 3.42-3.91 are pretty decent gears.
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Cowpatty69
Cochise

USA
226 Posts

Posted - 29 Nov 2017 :  9:46:44 PM  Show Profile  Reply with Quote
That calculator is kinda slick! Looks like at 3000RPM with 3.73s we're looking at roughly 67mph, 3.42s-73mph, and 3.23s-77mph. Should probably go 3.23s but will most likely be 3.42s (and hopefully not much highway time). Cortcomp- better keep that LS thing a secret on this site-lol!
Thanks!

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Phil
The Great White Buffalo

USA
7266 Posts

Posted - 04 Dec 2017 :  3:19:51 PM  Show Profile  Reply with Quote
Need the tire size. Unless those 70's are real tall, the 3.73's and automatic trans should put rpms around 3000 at 60 mph.

I'm 3000 rpms at 60 mph with 26" tall tires and 3.42 rear in high (1:1) gear. But I have OD so no its not a concern. I hated highway driving before I put the OD in though.

Bowties are for Pee-wee Herman. "Chevy": even the name sounds cheap, but not as cheap as your Pontiac will be with an LS transplant.
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Cowpatty69
Cochise

USA
226 Posts

Posted - 28 Jan 2018 :  9:36:42 PM  Show Profile  Reply with Quote
Update: People that set up rear ends are few and far between! Got the 3:42s in, and found a guy that sets them up, but he shared new info I thought I'd share. My original axle housing had a 2:41 gear, so that housing should stay in the 2:xx gear range. The 3:xx housings stay in the 3:xx family and so on. However, they make ring gear spacers to allow you to jump from like 2:xx to 3:xx. Specifically, my housing would go up to a 2:73, but need the spacer beyond that. My Ring and Pinion were from Summit and they also had the ring gear spacer- but not as many options as once available (people apparently do not do this much anymore). The spacer also comes with longer bolts and loc tight. I have also heard not a good idea for serious drag racing- which we won't be doing. The tech support at Summit was excellent in explaining this to me and the spacer is on order.

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67drake
Sitting Duck

Azerbaijan
1531 Posts

Posted - 29 Jan 2018 :  12:40:49 PM  Show Profile  Reply with Quote
I think the set up guy meant that the carrier is different from one series to the next. The housing would be the same if you were running a 2.41 or a 4.11,for example. A spacer is a way to run a numerically higher gear with a smaller numerically series carrier. But,as you mentioned,this is not a good idea with slicks and a high torque engine.


71' GTO Original 400 M20 3.23 posi
13.95@102.1 MPH on street tires
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Phil
The Great White Buffalo

USA
7266 Posts

Posted - 01 Feb 2018 :  8:53:46 PM  Show Profile  Reply with Quote
Yep, 8.5" corporate rears have always been 2.73-up carrier and anything below 2.73 has a different carrier with the flange set further back. Ergo the need for a ring gear spacer when using 2.73-up gears on a 2.41 carrier.

True the ring gear spacer is technically a weaker approach vs. switching to an actual 2.73-up carrier but (personally) I've yet to see one fail in street use.

Bowties are for Pee-wee Herman. "Chevy": even the name sounds cheap, but not as cheap as your Pontiac will be with an LS transplant.
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