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 Transmission / Driveline and Traction
 3.55 VS 3.73 Rear gear swap.
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79FORMULA400
Buffalo

USA
75 Posts

Posted - 25 Apr 2016 :  11:50:40 AM  Show Profile  Reply with Quote
Just curious as to what you guys think about the difference between the two. I currently have 3.08's in the 8.5" 10 bolt in the rear of my firebird, I plan on a gear ratio change very soon. I cant quite decide what would be the best for all around streetability, I feel like the 3.73s would be too much and turn too much RPM for the street. I have a Super T10 4 speed and a 29 inch tall tire in the rear (255/70/15) according to a Tire Height vs Trans Final Drive ratio Vs Rear gear calculator right now I run 2144 RPM at 60 MPH. I plan to gear it for when I move back home to southern VA (7.5 more weeks). The terrain there is much different than north western Ohio, I could probably stand to have a little more rear gear in the car. Due to the fact when I move back home my average cruse speed will drop from 70-75mph (Flat interstate) to 55-65mph. According to the calculator with a 3.55 gear it would raise my rpm at 60mph to 2468 and 2600 with a 3.73 rear gear. I would just like some input from the guys who have made the switch and your thoughts on the two. I also wonder about traction in first with all the extra torque going to the ground.(Although I have the lowest numerical first gear Super T-10 (2.43)) I just finished up the solid body bushings and sub-frame connectors yesterday, I might make a gear swap this weekends project.

1979 Formula 400 WS6, 1973 400 engine 4X Heads. Summit 2801 Cam, 8AN Fuel System,Edelbrock Performer MSD Dist, 6AL-2 Holley 750 Double Pumper,Super T-10, McLeod Super Street Pro, LSD Rear.

tjs44
Cochise

USA
413 Posts

Posted - 25 Apr 2016 :  1:17:12 PM  Show Profile  Reply with Quote
From my experience 3.55s are the max for a lot of street-highway driving.I actually went to a 3.31 and am very happy with it.Tom
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Blued and Painted
Chief PONTIAC

USA
3409 Posts

Posted - 25 Apr 2016 :  1:26:11 PM  Show Profile  Reply with Quote
I owned a GTO manual with a .373.. It's a great stop light to stop light gear. I don't recall ever being beat.

But when you get on the hi-way, a 4 door Audi will wax your azz.

Many Super Duty came with .342.. IMO, it's enough gear to roast the tire and enough legs for trips to Work or Dairy Queen. Again, Just my opinion.


Bull Nose Formula/ 461/ Q-Jet/
TH400/ 3.08 8.5 / R44TS.

Edited by - Blued and Painted on 25 Apr 2016 1:27:21 PM
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Blued and Painted
Chief PONTIAC

USA
3409 Posts

Posted - 25 Apr 2016 :  1:32:54 PM  Show Profile  Reply with Quote
.373 would be very street-able with an overdrive transmission.


Bull Nose Formula/ 461/ Q-Jet/
TH400/ 3.08 8.5 / R44TS.
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tjs44
Cochise

USA
413 Posts

Posted - 25 Apr 2016 :  1:41:05 PM  Show Profile  Reply with Quote
my 69 bird has a TKO500 and the 3.31 gets me at 65-70 at 2000 2100 with the 3.31s in 5th,LOVE them.They are very street friendly in 4th.Tom
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GtoGuy32
Cochise

USA
842 Posts

Posted - 25 Apr 2016 :  6:39:40 PM  Show Profile  Reply with Quote
I'm running 3.90s with 28" tires and a 4 speed.

I absolutely love this gear. I don't do a lot of highway. The car likes to cruise at 50-55mph.

With a 29" tire and 373, I think it will cruise nicely. I'd go for that over the 3.55, especially with a 400.
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phil400
Cochise

Canada
456 Posts

Posted - 25 Apr 2016 :  6:46:23 PM  Show Profile  Reply with Quote
I run 3.42's with a 27" tall tire, engine turns 3100RPM @75MPH, love this gear.
Took the car on a 450 mile round trip last summer.

78T/A 4-SPEED Marty blue 400/milled 6x-4's/268H,comp roller tip rockers/ performer Q-jet/RA manifolds 2.25" exhaust with x-pipe/ mine since '99 3rd & final owner as long as I'm alive
"sometimes I do what I want to do,rest of the time I do what I have to"
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GtoGuy32
Cochise

USA
842 Posts

Posted - 25 Apr 2016 :  10:21:50 PM  Show Profile  Reply with Quote
quote:
Originally posted by GtoGuy32

I'm running 3.90s with 28" tires and a 4 speed.

I absolutely love this gear. I don't do a lot of highway. The car likes to cruise at 50-55mph.

With a 29" tire and 373, I think it will cruise nicely. I'd go for that over the 3.55, especially with a 400.



Forgot to mention that I spend a lot of time at the strip with my car too. I think you will be happy with either gear you choose.
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79FORMULA400
Buffalo

USA
75 Posts

Posted - 26 Apr 2016 :  01:53:45 AM  Show Profile  Reply with Quote
Wow, Thanks for the responses! I am feeling like the 3.73 might just be the way to go here. I plan on magnum t56 gearbox swap next year (New job). I think going to the 3.73 would really help the off the line performance as is right now, and also help with the double overdrive of the T-56 in a year or too. I have also looked into the gear vendors setups, but at 2800 I could have an entire transmission that is rated for 700 Ft/lbs (And uses the same clutch, bellhousing).
From what I am reading here it all comes down to personal preference.
Back home in VA I am 30 minutes from the nearest 4 lane highway (With a speed limit of 55 none the less). Just all windy 2 lane backroads. I think the 3.73 will suit me just fine.

Thanks for the input!

1979 Formula 400 WS6, 1973 400 engine 4X Heads. Summit 2801 Cam, 8AN Fuel System,Edelbrock Performer MSD Dist, 6AL-2 Holley 750 Double Pumper,Super T-10, McLeod Super Street Pro, LSD Rear.
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loosetappet
Sitting Bull

113 Posts

Posted - 26 Apr 2016 :  07:15:46 AM  Show Profile  Reply with Quote
I run a 3.42 on my 69 Bird with a Muncie and 8.5 rear. 26.5 tall tires. This is a good combo for all around driving, very close to the factory supplied 3.36. For long trips I can switch to a taller tire for better mileage and slightly lower sound level.
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67drake
Sitting Duck

Azerbaijan
1501 Posts

Posted - 26 Apr 2016 :  10:35:55 AM  Show Profile  Reply with Quote
I don't think a 3.55 is available for an 8.5 rear,so you're between a choice of 3.73 or 3.42's. Personally I'd go for the 3.73's IF your going with a T56 transmission in the near future. Personally I'd go 3.90 or 4.11 with an overdrive,but...

I run 3.90's with a Muncie. No highway though and not a whole lot of locals can beat me in a race (not that I ever street race)


71' GTO Original 400 M20 3.23 posi
13.95@102.1 MPH on street tires

Edited by - 67drake on 27 Apr 2016 08:15:13 AM
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cortcomp
Coyote

USA
5338 Posts

Posted - 26 Apr 2016 :  12:01:57 PM  Show Profile  Visit cortcomp's Homepage  Reply with Quote
I ran 3.90s with OD, and it was still 3200+ RPM at modern highway speeds. No thank you. I think 3.73s are about right for a streetlight bruiser with OD and still good highway. If you don't mind the drone of aftermarket exhaust systems, 3.73s without OD.
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67drake
Sitting Duck

Azerbaijan
1501 Posts

Posted - 26 Apr 2016 :  9:51:05 PM  Show Profile  Reply with Quote
Yes,but the OP said he's not going to be driving at modern highway speeds. If I was driving on flat strait highway,@75-80 I would agree.


71' GTO Original 400 M20 3.23 posi
13.95@102.1 MPH on street tires
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cortcomp
Coyote

USA
5338 Posts

Posted - 26 Apr 2016 :  11:14:56 PM  Show Profile  Visit cortcomp's Homepage  Reply with Quote
ahh missed that. Then yeah, 3.90's would be fun fun around town. Maybe a medium revving 428 build?
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Phil
The Great White Buffalo

USA
7222 Posts

Posted - 27 Apr 2016 :  07:01:28 AM  Show Profile  Reply with Quote
...also a 29" tall tire.

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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79FORMULA400
Buffalo

USA
75 Posts

Posted - 27 Apr 2016 :  11:54:53 AM  Show Profile  Reply with Quote
quote:
Originally posted by 67drake

I don't think a 3.55 is available for an 8.5 rear,so you're between a choice of 3.73 or 3.42's. Personally I'd go for the 3.73's IF your going with a T56 transmission in the near future. Personally I'd go 3.90 or 4.11 with an overdrive,but...

I run 3.90's with a Muncie. No highway though and not a whole lot of locals can beat me in a race (not that I ever street race)


Hmm. I just looked on summit and sure enough a 3.42 is the closest thing you can get to a 3.55 for an 8.5" 10 bolt.
But the 3.73 should do just fine for what I am wanting out of it.

1979 Formula 400 WS6, 1973 400 engine 4X Heads. Summit 2801 Cam, 8AN Fuel System,Edelbrock Performer MSD Dist, 6AL-2 Holley 750 Double Pumper,Super T-10, McLeod Super Street Pro, LSD Rear.
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79FORMULA400
Buffalo

USA
75 Posts

Posted - 27 Apr 2016 :  11:59:20 AM  Show Profile  Reply with Quote
quote:
Originally posted by Phil

...also a 29" tall tire.


Exactly. The tire size difference provides a 300 RPM drop over a factory 26" tall tire with a 3.73 gear at 60 MPH.
Here is the calculator I am using. http://www.csgnetwork.com/multirpmcalc.html I believe Wallace racing has one of these as well.

1979 Formula 400 WS6, 1973 400 engine 4X Heads. Summit 2801 Cam, 8AN Fuel System,Edelbrock Performer MSD Dist, 6AL-2 Holley 750 Double Pumper,Super T-10, McLeod Super Street Pro, LSD Rear.

Edited by - 79FORMULA400 on 27 Apr 2016 12:01:54 PM
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Tub
Sitting Bull

172 Posts

Posted - 27 Apr 2016 :  12:10:09 PM  Show Profile  Reply with Quote
Back in the old days, I bought a new RA3 GTO, with a close ratio Muncie and 3.90 gears. I realize that it was a different day and time, and gas was cheap. But, I drove it over 60,000 street miles, before retiring it to strip only duties. 4.33 gears would have been better, for the track. But, for me, the 3.90's were perfect, for the street. I didn't do a lot of long trips, but did drive it to Galveston, TX. Back then, you could still drive right down the sand beach. There were several rows of parked cars, and a never ending stream of Musclecars parading up and down between the rows. It was really a cool scene. Glad I got to be a part of it.

Also drove it to New Orleans for Mardi Gras, one year. The trip was great. But Mardi Gras was the pits. I suppose I was expecting something glamorous, like Vegas. Instead, what I saw was a bunch of drunks and sleezy bars with carnival barkers outside trying to intice those drunks into the bars. And this was Bourbon street. I expected a higher class deal. There were horse mounted police, patrolling and filling up the paddy wagons, which were parked on every corner of Bourbon. When a wagon got full of drunks, they'd take 'em to jail, and park an empty wagon, for the next load. That was probably in 1970. I'll never forget it, and I've never wanted to go back to Mardi Gras.

Well hey, I won't charge ya'll nothin extra for that little bit of history. Anyhow, the speed limit was 55 on most of the roads I traveled back then, so the 3.90 gears seemed perfect to me.

Edited by - Tub on 27 Apr 2016 12:12:24 PM
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