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Discussion Starter · #1 ·
Ok , I gotta do a gear set THIS WEEK ,actually like "yesterday" , it's a terrible thing to drive.

Looking for some feedback on what rear gear everyone has (and is really satisfied with) with an M21 that's great around town and great off the line , but still driveable on the highway? Not going to the drag strip or going to abuse the little 10'er though , so please don't recommend going to 12 bolt , I've got to get a gear in and done with the differential I have.

What is the max (lowest) gear ratio usable with a 2 series carrier ? (I've got a 10 bolt 2 series posi unit sitting here to use)

Can you use the 2 series (ED 32118) with a spacer for a 3.70's or 3.90's ? or is it the other way around.

Anyone have any good differential information websites ?

Gotta get this 2.56 outta there , it almost burnt up my new Centerforce this weekend trying to inch around in traffic. Talk about a horrible combo with an M21.....(although I can run 75 in second gear !)

(please spare the "demon" ring gear spacer , bolt shearing argument)

Thanks,

Brian
 

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I'd go with the 3.55. freeway friendly, and it'll still be a kick in the *** to drive after lugging around something as tall as 2.56. I have 3.55 in my car and I rev 3000 on the freeway, so I'm not over revving by any means, and it's still night and day between this and the 2.73's I used to have.
 

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RSSS69conv,
i had a 2.56 in my 80 camaro (10 bolt too) and went to a 3.73
acceleration is awsome and i am building a weekend warrior with higher than stock horsepower (400+)
however, i dont reccomend them for the highway, i turn about 3000 RPM at about 60mph
but for highway (and gas mialage) i would reccomend no steeper than a 3.55
and going from the 2.56 it will be a kick in the pants
bad news is that i had to replace the carrier and you will to if you are going into the 3's as far as ratio's go
i dont think that there is any other way...
good luck
Nate
 

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Your real problem is the M21. It is like starting out in second gear with most trans. Give serious thought to a 3.42 low ST10 or even a 3.50 or 3.11 low Saginaw.
 

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I like my M-20 and 12 bolt 3.31's but don't know what their equivalent is in the smaller rearend. And you prob. have to switch to a three series carrier but again, don't know for sure. Hope you find a set you like and can live with. Good luck. pdq67
 

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3.70's in my nova with a 26" tall tire..2800 at 60...not too bad, really.just this weekend i drove 135 miles one way(270 total) down interstate at 75 mph, crank the stereo and it is fine. got 16 mpg..

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1971 Nova(looks like 69 camaro from underneath!)
355sb, vortec heads, HOT cam,T-10 tranny, 3.70 gears 16" IROC wheels
 

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I've got 3.31's in mine. I run at 3,000RPM at 60mph with the tires I have. If you drive on the highway don't go any lower than 3.55.

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'69 RS SS 396 375hp L78 Convertible, M22, 3.31 posi, deluxe interior, gauges, rosewood wheel, AM-FM,fold down seat, tilt wheel, Torque Thrust Wheels, console mounted 8 track player, chambered exhaust
http://albums.photopoint.com/j/AlbumIndex?u=1169124&a=8603468
 

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I have an M21, I used to have a 3.42 posi (looking for another). It was great, and I could still drive on the highway, but I did avoid it and kept it below 65 MPH.
 

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One thing to consider is what size engine/ how much torque will it put out at low end. I have a 283 w/M-21, 4:56 posi and 27 inch tires. I can't spin the tires without dumping the clutch at 3K. Not really a bad thing but you need to be real careful letting the clutch out. I Plan to go to a 350 w350-400 Lb/ft of torque around 375hp, and 3:73 gears. I hear 3:73's are about it unless you go 5 speed.
 

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You don't say what size tire you're running but I'd say a 3.55 would be nice. I have 4.10's with an M20 and I can start in 2nd gear and, at highway speeds, it's pretty busy.

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Rick Dorion
69 RS Conv,355,M20,4.10's and I don't worry about stone chips.
 

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In two four speed cars I own, one is a 3.07 and the other is a 3.73. They'be both 69's and both have M-20's with a 2.52 first gear. Huge diff in acceleration and responsiveness.
Some difference in hwy noise. Your M-21 has a 2.20 1st gear...more for road racing.

This is all a matter of preference. If you're in to testing the ol' headrest on occasion, go with the 3.73's. If you don't want to hear your drivetrain sing on the freeway, go with something in the lower threes.

For acceleration total the sum of your first gear to your rear ratio and shoot for something in the 6 range (i.e. 3.73 + 2.20 = 5.93). For highway comfort, add your fourth gear (1:0) to the rear ratio and shoot for a sum of 3. For compromise, find something in the middle. Remember, that first gens originally used 24 inch tires except for some Z's. Today most of us are using 26 or taller which will turn a 3.73 into a 3.55...make sense?

Let us know what you decide and how it turns out.
 

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Discussion Starter · #13 ·
Thanks everyone.....

To answer the ques. about tires size , they're 235 60 15 Goodyear Nascar Eagles. They measure 25.25 tall (installed).

Right now I think I have it narrowed down to 3.73 or 3.55.

I think what I might do is the 3.55 to make it more drivable for the rest of this season since there's a lot more shows we want to attend , then at some point I think I'll stick an M20 in. That way with the 3.55's it might be a little closer to acting like 3.73's in first gear around town but still have the 3.55 end result on the highway.

Any better idea's ?
 

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Here's another [expensive] way to go, that might allow you to have a higher number gear ratio in the back & still drive it on the highway...

An overdrive that hooks up after the transmission. http://www.drivetrain.com/overdrive.html

Here's the page GM automatic & some manual transmissions:
http://www.drivetrain.com/GMC2wdapplicationguide.html

Here's somebodies opinion on one... http://www.rverscorner.com/info/auxtrans.html

Anybody here have one?


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David
Camaro - '68 327 Coupe, '86 Z-28 IROC 305 TPI
Corvette - '73 Mako Shark II, '79 L-82, '82 Cross-fire, '01 Coupe


[This message has been edited by djunod (edited 09-10-2001).]
 

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I swapped out the original factory 4.10 in my (stock) '69Z with an M-21 for a 3.55 last summer - works great - OK around town, now only revs 3200 at 70 (instead of 3850) with 26" BFG 235/60-15 TA's.

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JohnZ
'69 Z28 Fathom Green
 
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