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Old 03-07-2012, 08:53 AM   #1
CWerner
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Default 3.55 vs 3.08 rear gears??? and how dificult of a swap is it?

Okay, so on my quest to get better SOTP feel in my 72 Coupe, I'm working on many things. Here's what I have in case you haven't seen my other posts:

1) Original engine rebuilt, bore 40 over, turned crank, standard engine kit rebuild.
2) Edelbrock Performer 2101 intake with NEW Edelbrock Q-Jet style carb with electric choke with Mallory pointless Distributor
3) Ram Horn exhaust tied to new 2" rear exhaust...quite mufflers
4) Rebuilt 400 auto tranny

So I'm working on the tune to make sure it's right. I've decided to keep the car as original "looking...aka stelth" as possible.

So, I'm wondering two things:

1) Who has changed out the rear end gears to a 3.55 and how did you like it? I'm running 75 mph @ 3000 rpm. What does that chnage to with these gears? How much SOTP improvement did you get for normal (non-hwy) driving?

2) Difficulty factor for DIY rear replacement? .....or should I have a trusted source do it?
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Old 03-07-2012, 09:52 AM   #2
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I wouldn't do it, especially the way gas is going. Rear end work isn't for the amature, as requires proper dial gauge measurments and shimming to get things right. I changed from 2.73 to 3.73 gears in a car, and while off the line was better, did not like highway cruising at all. Felt like I driving in second gear. Has a 700R4 od tranny in it now. In addition to the 0.70 od lock up on these tranny's, 1st gear's also a 3.05 ratio. That will get you more off the line dig, than the 3.55 ring/pinion change you're talking, plus great highway cruising. If you're still really wanting to do the rear ratio change, I'd hunt down a whole nother assembled diff, and do it that way.
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Old 03-07-2012, 12:47 PM   #3
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I aggree it's not a DIY for someone inexperienced. The change though would be quite noticeable SOP wise and your RPM/MPH would be about 70mph at 3K.
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Old 03-07-2012, 01:03 PM   #4
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At any speed, engine RPMs will be about 15% higher with the 3.55 gears, compared to 3.08 (3.55 / 3.08 = 1.15). Also, as others have said, installing the gears requires experience, special tools etc.
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Old 03-07-2012, 01:09 PM   #5
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I did mine recently. It's not hard to do but you need to really educate yourself. Also you need a few measuring tools, aka dial indicator and micrometers. A inch pound dial torque wrench or access to 1. And a big breaker bar ( i used a 4 ft bar) to crush the new pinion sleeve, and a device to hold the the pinion yoke while torqueing the pinion nut. So if you have those tools accessible, you can do the job on your own. I borrowed most of the tools necessary from work. The job itself is straight forward.
Go to a web site like randysringandpinion for an online calculator to see what gear may be of use. 3.55 may be too much gear, a calculator let's you input different criteria and gives you what your rpm will be at different speeds. Read as much as you can before tackleing the job.
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Old 03-07-2012, 01:30 PM   #6
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I can add maybe another thing. In a car I transplanted a 3.73 gear into, about 3500 rpm @ 70 mph, with converter slip. The converter wasn't stock, but nothing to radical. About 2500 rpm now, using a 700R4 with lock up converter. Really like this set up.
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Old 03-07-2012, 01:34 PM   #7
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I had 3.55 gears in my Mustang, which also has a 1:1 third gear ratio. (same as hydromatic turbo 350) I never liked going long distances on the highway. The engine was really humming to keep up with 70 mph traffic.
I would not recomend the 3:55 gears if you're doing any serious highway driving Unless you switch to an overdrive transmission also.
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Old 03-07-2012, 01:34 PM   #8
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Default Trans Swap..

This is out from left field but how about a trans swap.My car has 3.08 rear gears and a Richmond 6 spd trans.The gear spreads are perfect for that rear gear and the car is a blast to drive from the old TH350 that was in there before. 1st gear is equal to a 4.11 rear and the 6th gear keeps the rpm's down on the highway.Best of both worlds.[IMG]Click the image to open in full size.[/IMG]
If you still want to change rear gears contact GTR1999 he is the specialist when it comes to rear gear changes.Send your diff to him and presto new rear gears.This is a job you want a pro to do for sure....Good luck.....
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Old 03-07-2012, 02:13 PM   #9
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If you have the tools and the time, and you're willing to read up and learn from guys like the aforementioned GTR1999, you can do your own diff rebuild/gear swap. If you don't have all the tools and aren't looking for an excuse to buy them, I think you'll find it's cheaper to send it off to a rebuilder.

I have a TH400 and changed my rear gears from 3.08s to 3.73s. I really like it, but I don't drive on the highway much, or drive with the top up much. If I had the dough I would get a 200 4R, would be a good combo with the 3.73s
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Old 03-07-2012, 02:22 PM   #10
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I would just try and find a good used 3.55 rear diff and swap it in. they pop up in the for sale section every once and awhile.
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Old 03-07-2012, 03:38 PM   #11
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Great input!!

So I don't think I'll do this myself. I do some limited hwy driving, but not enough to worry about the rpm level. It's mainly Houston stop & go hwy driving. The gears would help that issue.
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Old 03-07-2012, 07:10 PM   #12
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My 63 had a 302 with a wide ratio 4 speed trans and a 3.08 when I got it. It was a dog out of the hole and seemed to fall on it's face between gears unless you had it wound up really tight. My first step was plugs, wires, cap, rotor, and performance recurve of the distributor. This made a notiucable difference but still had the gearing issues (302 is not a torque monster). Next modification was a rear end swap to a 3.55. It totally transformed the car as far as SOTP feel. It feels like a 15% increase in HP plus it gets into the engines powerband quicker. Night and day difference. Mike Dyer, (Tracdogg2) is in Texas. Him and GTR 1999 are the two rear end guru's on the forum. He can fix you up

Last edited by 63mako; 03-07-2012 at 07:13 PM.
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Old 03-07-2012, 08:55 PM   #13
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I made the change and love it. Had the same set up, 350/th400/3.08 and was a dog off the line with my new cam.

First I put in a 2800 rpm torque converter. that made a small difference. It was like riding the clutch .

I bought a used differential on craigslist. It needed new axle seals but seemed fine. To swap the whole differential there is less need for micrometers and more need for sledge hammers. If you have air tools and a good flor jack you could do it.

It made all the difference to my car. 0-80 mph is a transformed car and car will spin tires at start now if I want. I am looking for a 2004r though.
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Old 03-07-2012, 10:46 PM   #14
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For the cost off changing the rear end (if you have someone else do it) you can drop in an OD tranny . Your rear end will 'think' it has 3.80 gears in it off the line yet on the highway you'll be reving at 1800 rpm at 60 mph.
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Old 03-07-2012, 10:49 PM   #15
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I've had taller and deeper gears from 2.72 to 5.13, but IMHO the 3.55 is hard to beat as a general purpose performance ratio. For my particular purposes I wouldn't have any other one in my shark. My $.02

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Old 03-08-2012, 11:32 AM   #16
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You will be turning 3200 rpm at 70 mph with the 3.55 ratio. As for seat of the pants feel it will be a giant change.
Looking for a used 3.55 is not a good idea. Because of the tooth count of the oem gears,9-32, these gears run hot and wear fast. The new gears , 11-39, are much better. Everyone makes a 3.55 ratio so you have several choices of manufacturers.
There's alot more involved inside the diff that needs attention besides a gear change. Casting imperfections in the carrier need to be addressed. Setting up the posi plates is vital for getting the correct spider gear backlash. A steel cap is always a good idea, even on a stock diff, but not mandatory. Machining the pads for the caps is mandatory. I have yet to see a diff housing that has the pads machined square.
Mike
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Old 03-08-2012, 12:21 PM   #17
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Your ideal performance first gear final drive ratio is 10.0 to 1. Your existing setup is 7.64 to 1. With a 3.55 your first gear final ratio will be 8.80 to 1. Noticable difference. With a 3.70 rear gear your first gear final drive ratio would be 9.17 to 1 and you would be running 3335 rpm @ 70 MPH
If you decide to convert to a 200 R4 later a 3.70 rear gear will have a 10.13 to 1 first gear and the final drive in OD is 2.48 to 1. They are a relatively easy swap and are available with a lock up convertor. I would go with the 3.70 for your objective. If you decide to go with a 200 R4 OD later you have the correct gearing and great fuel mileage and cruise comfort. If you decide to upgrade to a tko 600 manual the 3.70 works well with it also with a 10.62 first gear. The 3.55 and TKO 600 gives you a 10.19 first gear final drive. I depends on where you ultimatly want to end up as far as drivetrain upgrades, comfort level and fuel mileage desired at cruise.

Last edited by 63mako; 03-08-2012 at 01:43 PM.
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Old 03-08-2012, 03:56 PM   #18
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2 years ago I switched from 3.55 to 3.08. I was tired of the engine screaming when on the highway. Much better now.
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Old 03-08-2012, 04:39 PM   #19
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I have a 3:55 rear and its great,it is the best performance all around gear,any mechanic will tell you that.Yes it does rev a little going 70 mph,but its not that bad,i had 4:11,4:56 gears and they were bad on the belt way. You will be happy with 3:55 gears,guarantee it. Remember ,3:55 is the best" ALL AROUND" gear ratio for you. A 3:08 gear makes my car run like a turd.Its just good for high mph driving.And saving gas i suppose.

Last edited by Billysvette; 03-08-2012 at 04:41 PM.
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Old 03-11-2012, 06:54 AM   #20
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SOTP...... heated seats................snicker, snicker..... I couldn't resist... aw, don't hate me, just kidding.
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Old 03-11-2012, 06:54 AM
 
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