Changing differential ratio
Thread Starter
Melting Slicks
Joined: Mar 2006
Posts: 3,342
Likes: 7
From: Connecticut
Changing differential ratio
Hi all
I have a '73 coupe with a 3:08 rear in which I installed a ZZ4 a few years ago. I have the original numbers matching L-48 buttoned up under my work bench. The Muncie M20 transmission and the 3:08 differential are original as well.
I would like to change the gearing to 3:36 or 3:55 as the 3:08 is quite depressing to drive with the ZZ4.
My question is can I still use my housing and have lower gears installed or do different ratio differentials require different housings?
If I can use my current housing would anybody have a guess on a ball park figure of what the cost of upgrading might be and where could I get the work done by someone with a good reputation?
In the event I can not use my original housing would anybody have an idea what a good used 3:36 or 3:55 used unit would go for?
Also, any recommendations on the 3:36 vs 3:55 would be extremely helpful since I cant just go and test drive something with the same drive train that I have in mind.
.
I rarely drive more than 30 minutes on the interstate and I'm not concerned about fuel mileage. Most of my driving is around town and on secondary roads.
Thanks for any info or suggestions.
Bob.
I have a '73 coupe with a 3:08 rear in which I installed a ZZ4 a few years ago. I have the original numbers matching L-48 buttoned up under my work bench. The Muncie M20 transmission and the 3:08 differential are original as well.
I would like to change the gearing to 3:36 or 3:55 as the 3:08 is quite depressing to drive with the ZZ4.
My question is can I still use my housing and have lower gears installed or do different ratio differentials require different housings?
If I can use my current housing would anybody have a guess on a ball park figure of what the cost of upgrading might be and where could I get the work done by someone with a good reputation?
In the event I can not use my original housing would anybody have an idea what a good used 3:36 or 3:55 used unit would go for?
Also, any recommendations on the 3:36 vs 3:55 would be extremely helpful since I cant just go and test drive something with the same drive train that I have in mind.
.
I rarely drive more than 30 minutes on the interstate and I'm not concerned about fuel mileage. Most of my driving is around town and on secondary roads.
Thanks for any info or suggestions.
Bob.
Le Mans Master
Joined: Apr 2006
Posts: 5,403
Likes: 95
1966 up M-20 ratios 2.52,1.88.1.46,100. X rear ratio's
7.76,5.79,4.49,3.08
8.46,6.31,4.90,3.36
8.94,6.67,5.18,3.55
9.32,6.95,5.40,3.70
10.35,7.72.6.00,4.11
Having owned 3.08, 3.36, 3.70 ,4.11 with M/20, M-21. Borg warner's ST10s with 2.43 close ratio X 3.70. 2.64 wide ratio X 3.73. Tire dia. ranging from 26" to 29"
Hydraulic roller 350 along with wanting mostly intown accel. with limited freeway driving. Would recomend the 3.70 with 27" tire dia down to 26" dia. If you have less then 26" dia. the 3.55.
3.36 would be so little help to you it would not be worth the cost of a gear change you would be real dissapointed in the difference.
7.76,5.79,4.49,3.08
8.46,6.31,4.90,3.36
8.94,6.67,5.18,3.55
9.32,6.95,5.40,3.70
10.35,7.72.6.00,4.11
Having owned 3.08, 3.36, 3.70 ,4.11 with M/20, M-21. Borg warner's ST10s with 2.43 close ratio X 3.70. 2.64 wide ratio X 3.73. Tire dia. ranging from 26" to 29"
Hydraulic roller 350 along with wanting mostly intown accel. with limited freeway driving. Would recomend the 3.70 with 27" tire dia down to 26" dia. If you have less then 26" dia. the 3.55.
3.36 would be so little help to you it would not be worth the cost of a gear change you would be real dissapointed in the difference.
Last edited by Little Mouse; Jul 13, 2011 at 03:00 AM.
Le Mans Master
Joined: Sep 2004
Posts: 5,116
Likes: 5
From: Livonia MI
I'd change the transmission instead. You didn't mention the transmission you have, but if current 1st gear ratio is 2.50, moving to a 700R4 or TKO 5 speed will be similar 1st gear to running a 3.73 axle. Not to mention the 2.20 equivalent highway cruising ratio. Ideally, even with the tranny change, would probably desire a 3.42 axle, but after coughing up for the O.D., rear end work isn't cheap either.
Race Director
Joined: Dec 1999
Posts: 19,610
Likes: 778
From: Forked River NJ
Yes you can still use your housing. The 3.08 and the 3.55 use the same interanl components except the ring and pinion is different. You have one of the best rebuiulders in your backyard, Gary Ramadi is in the New haven area. He has rebuilt lots of C3 rears for forum members. The 3.55 is an excellent overall ratio even if yo udo change to a 5 speed trans. I actually have a spare 3.55 in my garage that will probably go into a 5 speed swap in the future for someone. I pick these up when I can get them. The cost is going to depend on what is needed, obviously new ring and pinion. The 3 series carriers will support a 2.73-3.90 gears and there are special thick flange 4.11 gears also available for the 3 series carrier.
Drifting
Joined: Jul 2008
Posts: 1,483
Likes: 459
From: Lititz PA
The 3.55 ratio with your M20 wide ratio gear box would be a nice, conservative combination. Going from 3.55 to the 3.73 ratio will add another 5% to your RPM, Torque, and Fun! Don't waste your time on the 3.36, you'll hardly notice the difference. If you are considering the 4.10 ratio, personally, I enjoyed the 3.73 more than the 4.10's. With the 4.10 gears, you barely get going, and you have to shift right away. Of course, you'll get going much quicker too!
John
John
Race Director
Joined: May 2006
Posts: 16,528
Likes: 53
From: Dayton, Ohio
Hey Bob , remember this one ? It is on the conservative side,the owner came up with the combo. With just 4 gears and the low end torque of the ZZ4 it is a pleasure to drive. It can come from a dead stop burning the tires BUT will also accelerate nicely from 35mph while in 4th gear.
Thread Starter
Melting Slicks
Joined: Mar 2006
Posts: 3,342
Likes: 7
From: Connecticut
Wow, Thanks for all the great reply's.
I forgot about Gary, he's only about 20 minutes from me so I'm going to contact him and see and see if he can do the job for me.
From the info posted here I'm leaning towards the 3.55
I'll let you guys know how I make out.
Thanks very much to all for all the help.
Bob
I forgot about Gary, he's only about 20 minutes from me so I'm going to contact him and see and see if he can do the job for me.
From the info posted here I'm leaning towards the 3.55
I'll let you guys know how I make out.
Thanks very much to all for all the help.
Bob
Thread Starter
Melting Slicks
Joined: Mar 2006
Posts: 3,342
Likes: 7
From: Connecticut
Hey Bob , remember this one ? It is on the conservative side,the owner came up with the combo. With just 4 gears and the low end torque of the ZZ4 it is a pleasure to drive. It can come from a dead stop burning the tires BUT will also accelerate nicely from 35mph while in 4th gear.
I'm just shocked that the engine you installed for him hasn't fallen out yet
Just funnin with ya buddy!
You do great work!
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Thread Starter
Melting Slicks
Joined: Mar 2006
Posts: 3,342
Likes: 7
From: Connecticut
