Rear Differential change
Thread Starter
Instructor
Joined: Sep 2010
Posts: 157
Likes: 0
From: Rockwood TN
Rear Differential change
I'm going to be replacing my 3.36 rearend with a 3.55 rearend.I have a 475HP ATFW,Super T10 4 Speed tranny.Has anyone by chance made this swap before or anyone know what I can expect with this change?I'm wanting alittle more performance out of her, but as far as gearing goes,I don't want to go radical like a 3.90 or 4.11.Any Ideas on what I should expect? 
Last edited by bobs76stingray; Jul 1, 2012 at 08:24 PM.
Pro
Joined: Apr 2009
Posts: 650
Likes: 1
I'm going to be replacing my 3.35 rearend with a 3.55 rearend.I have a 475HP ATFW,Super T10 4 Speed tranny.Has anyone by chance made this swap before or anyone know what I can expect with this change?I'm wanting alittle more performance out of her, but as far as gearing goes,I don't want to go radical like a 3.90 or 4.11.Any Ideas on what I should expect?
Don't expect much of anything with that swap. Go up to a 3:70 ratio.
Le Mans Master
Joined: May 2002
Posts: 5,086
Likes: 786
From: Lebanon Township New Jersey
It boils down to simple arithmatic. 3.55/3.35 = 1.059. So at a given speed (correcting the speedometer of course) in a given gear your rpms will be 5.9% higher. Not worth the effort in my opinion.
Le Mans Master
Joined: Apr 1999
Posts: 6,134
Likes: 46
From: Raleigh North Carolina
I'm going to be replacing my 3.35 rearend with a 3.55 rearend.I have a 475HP ATFW,Super T10 4 Speed tranny.Has anyone by chance made this swap before or anyone know what I can expect with this change?I'm wanting alittle more performance out of her, but as far as gearing goes,I don't want to go radical like a 3.90 or 4.11.Any Ideas on what I should expect?
I agree with the 3.70's anyways. That would be my pick bang for the buck.
Thread Starter
Instructor
Joined: Sep 2010
Posts: 157
Likes: 0
From: Rockwood TN
Thanks Tom454,I've been debating between the two,That helps alot.And its a 3.36,I just hit the wrong key.This is just one area of a car that I've never been involved with.Thats why I'm asking advise.Thanks for your help.
Burning Brakes
Joined: Jun 2010
Posts: 1,165
Likes: 93
From: Bellmore NY
I am in the same situation, as I have 3.36's, M21 (rebuilt as an m20), and I have a 3.55 set up ready to go in.
Am I expecting a huge difference? No, but the unit going in is fresher than the tired one in the car and the new one was a good deal. To do alll the work without ANY gain would have been pointless to me, but I refuse to step beyond what I feel to be a reasonable compromise for street and highway.
Its not my everyday car, but Ive had 3.73 and 4.10 gearsets before, and that is not in the "compromise" category. 3.55 is as far as Im willing to go, and I think the overall driveability, and general feel of the car will just be "better" than the 3.36s with a nominal tax paid in highway speed.
Think of it as just finding a more comfortable shoe. Maybe the shoes you are wearing right now are nice, but with a lot of effort/expense you can be wearing a shoe that is marginally better but "just right". Doesnt really sound like its worth it when you look at it like that, but to me, and at my age, I see gearing as something that is real easy to go overboard with.
Am I expecting a huge difference? No, but the unit going in is fresher than the tired one in the car and the new one was a good deal. To do alll the work without ANY gain would have been pointless to me, but I refuse to step beyond what I feel to be a reasonable compromise for street and highway.
Its not my everyday car, but Ive had 3.73 and 4.10 gearsets before, and that is not in the "compromise" category. 3.55 is as far as Im willing to go, and I think the overall driveability, and general feel of the car will just be "better" than the 3.36s with a nominal tax paid in highway speed.
Think of it as just finding a more comfortable shoe. Maybe the shoes you are wearing right now are nice, but with a lot of effort/expense you can be wearing a shoe that is marginally better but "just right". Doesnt really sound like its worth it when you look at it like that, but to me, and at my age, I see gearing as something that is real easy to go overboard with.
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Race Director
Joined: May 2005
Posts: 10,674
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From: Millington Illinois
St. Jude Donor '08-'09
It will also "feel" like 6% more HP. or 503 hp with less drop between gears and better launch. I think you will notice that and not give up your cruise rpm. I had a ST10 in my vette when I got it with a 3.08 behind a 325 hp 302. Changed to a 3.55 and it transformed the car. The ST10, 3.55 combo was a perfect match for power and cruising. If your willing to sacrifice the cruise RPM a 3.70 will get you more seat of the pants difference.
Last edited by 63mako; Jul 1, 2012 at 11:40 PM.
Melting Slicks
Joined: Aug 2008
Posts: 2,200
Likes: 3
From: Lehigh county Pennsylvania
Your engine will be turning 3-3200 rpms at highway speeds with a 3.70 rear. Lots of folks on this forum are more inclined to chug along at a lot lower rpms. I think a swap to 3.55 would be a good compromise for you.
Le Mans Master
Joined: Apr 1999
Posts: 6,134
Likes: 46
From: Raleigh North Carolina
I'm going to be replacing my 3.36 rearend with a 3.55 rearend.I have a 475HP ATFW,Super T10 4 Speed tranny.Has anyone by chance made this swap before or anyone know what I can expect with this change?I'm wanting alittle more performance out of her, but as far as gearing goes,I don't want to go radical like a 3.90 or 4.11.Any Ideas on what I should expect?