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?

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.

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.

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.

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.

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.

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.

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]http://i940.photobucket.com/albums/a...i/100_1538.jpg[/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.....

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

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.

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.

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:thumbs:

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.

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.

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

:cheers:

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

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.

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.

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.:ack: 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.

SOTP...... heated seats................snicker, snicker..... I couldn't resist... aw, don't hate me, just kidding.

As other posters have mentioned, your actual type of driving will dictate your choice. I also believe your age will make a big difference on what you will find acceptable.

Much like other posters Ive had a wide range of axle ratios in different vehicles. Ive had 3.08, 3.36, 3.42, 3.73, 4.10, and 2.73 with od.

When I was 20 I had a car with 3.42s, and it seemed ok on the highway.
When I was 21 I had a car with 3.73s, and it seemed ok on the highway.
When I was 25 I had a car with 4.10s, and it was tolerable on the hwy.
When I was 35 the car with 4.10s was not pleasurable anymore.

I will be 37 this year and my 3.36 vette seems to be a decent compromise all around, even though sometimes even this gearset seems a little restrictive on the highway. 10 years ago I would have said that my future [present] self is a wuss.

I think its age, and exposure to the comfort of more modern cars that makes it harder to put numerically higher gears in a car.

Think we're all just getting spoiled now, and with today's availible tranny's and gas prices, why not. My daily 08' Pontiac GXP 3.6L, 6 speed auto, will basically trounce most C3's, and about 1800 rpm @ 70 mph, getting 28 mpg. My C6 M6, is about 1500 rpm @ 70 mph, with similar fuel milage to my daily car. Today's tranny's are the real stars. Why not take advantage of them. As mentioned earlier, have another car originally with a TH-350. Now has a 700R4 with lock up od, and the cat's meow.

I went from a 3.08 to a 3.55, and it was a big difference off the line. However, I had already changed the 400 auto to a 700R4.

FYI: If I would have known about the 2004R transmissions I would have done that instead for the closer 1-2 gears, but the 700 does fine.

Okay, so here's another option/question.

What if I leave the 3.08 rear and add a 700r4? I've got one in my C2 that I've been considering swapping out for a TKO.

I just love it with my TH400 and 3.08 gears when both the needles are straight up at 80 mph and 3 grand:thumbs:.

It will feel better in first gear only but you will be lugging the engine on the highway. 3.08 is too tall of a gear for an od. 3.55 or 3.73 is best for an od. It gives you the best of both worlds, acceleration and economy. Just going to a 3.36 would be a big help.
Mike

Thanks to all.

I think my plan will be to look for a used 3.55 rear end and do a swap.

Then I'm set for any trany swap out, whenever I decide to do it. If I don't, then I've still got the SOTP feel I was looking for.

So, did you ever find your 3.55? I'm looking for the same swap and wondered what you ended up doing.... I have a forum member that is interested in a 3.73 swap for my 3.08that would be great off the line but beyond that a bit harsh, especially with headers / sidepipes.....

Looking to swP A 4.11 for a Chevy 3.55 gear differential set up
 

Where do I find these 3.55 rear ends? I am willing to horse trade my original 4.11 factory 6 lug out of my 64 truck. I have already put new 5 lug disc on front. S this gives the added value of having 5 lug in the back. Now I can rotate my tires and get good highway performance.
Tks for any help - Steve

I want to swap pout my original 64 chevy truck differential 4.10 fro a 3.55.
 

Help Needed: - I want to swap out my original 64 6 lug chevy truck differential 4.10 for a 5 lug 3.55. Where do I find a complete axle to axle unit? What year model gms / chevys have this 3.55?
Tks - Steve

You have a truck rear end in your Corvette?

Old Chevy Truck Driver:
Do you own a corvette? if so what year? How can you have a truck differential in a C3 corvette that comes from the factory with a independent rear suspension? There is no axle to axle on a C3 corvette.

Sorry but no - I am new to this posting stuff - Sorry.
 

I guess this is where i am supposed to put a new Post?????????? First day- so sorry yall. Anyway I am trying to find a swap/trade/buy/sale something to switch out my original 1964 6 cyl 6 lug Chevy 12 bolt rear end, to a 5 lug 3:55 gear ratio. My understanding is that my original is a 4:10 or a 4:11 gear ratio. and I think PT also???? Runs great now, no road noise or whining. So what Makes and/or Models can I look for in the barns and weeds to get me going? I hear go back to a 10 bolt. Any horse trading out there? Too difficult for beginner to change out all the hog head, axles, brgs etc. Tks -Steve

Sorry but no - I am new to this posting stuff - Sorry.
 

I guess this is where i am supposed to put a new Post?????????? First day- so sorry yall. Anyway I am trying to find a swap/trade/buy/sale something to switch out my original 1964 6 cyl 6 lug Chevy 12 bolt rear end, to a 5 lug 3:55 gear ratio. My understanding is that my original is a 4:10 or a 4:11 gear ratio. and I think PT also???? Runs great now, no road noise or whining. So what Makes and/or Models can I look for in the barns and weeds to get me going? I hear go back to a 10 bolt. Any horse trading out there? Too difficult for beginner to change out all the hog head, axles, brgs etc. Tks -Steve

This forum is for Corvettes. You'll have better luck here http://www.67-72chevytrucks.com/vboard/

Tks
 

The only reason I went here is I saw multiple post on 3:55 gears. But yes I will move over to the truck world. Tks again- Steve

That is a great forum, I am a member there as well.

Sorry, but if you don't know the diff between a Corvette gear and a truck gear you might be up for a little reading on both first.

Trucks were generally made for hauling and pulling, thus the stiff rear gear. Not for sure if a highway gear could be ordered, but it would probably be somewhat rare.

3.36 200 r4 roller engine 350, serp drive....electric fans, DPFI, and happy as a clam.....

~2400 rpm at 80 cruise........and I get passed fairly regularly....

but it's lesser cars trying to prove a point......


:D:thumbs:

What are you talking about?

I think he means back before trucks commonly had V8s, they usually had low(high numerical) gears to make up for lack of power.

that is right. i have a 81 vette with a 510 hp 400 blueprint engine, with 3.73 gear 4l60 trans & it runs good on the highways

you want to do similar to what i done, i have a 510 hp blueprint 400 engine, with 3.73 gears, a 4l60 4 sp trans in a 1981 vette. good bottom end & runs good on the highway.

There are so many ways to accomplish what you want. Here is a spreadsheet I built to let everyone play with gear ratios and trannys.

The real problem was the old close-ratio trannys only had a 2.2 gear spread between 1st and top gear. That meant a 3:55 was a good all-around compromise gear. But the OD trannys can go to as high as 6.0 gear spread. That means you can have the best of both worlds, great acceleration and comfortable low rpm cruising.

My 72 BB had a very low 7.2 to 1 overall in 1st gear. Horrible combo, 2,20 box and 3.36 rear. Feels like you are starting in 2nd gear and revs to 60+mph in first. Cruises fine though.

My LT-1 had both a CR/4.11 and a WR/3.55 in it at different times and both were great in town. 9 to1 overall, it's about the minimum to feel "good" off the line. But only the 3:55s felt tolerable on the highway. 60mph was a comfortable cruise with a free-revving LT-1 at 2700 at 60, 3200 at 70. But even with that gear at 70+ the oil tended to heat up after a 5-10 min and the oil psi would drop, and all I was trying to do was to keep up with traffic! So it was tolerable on the highway but not ideal.

My current build will have both a 10:1 off the line and a 2200 rpm cruise at 70mph with a CR TKX OD, and a 3.36. But it will take the 4.2 gear spread of a TKX to do that.

gear ratios
 

on my 1970 big block restomod I am keeping the 3.08 first gear but going with the wide ratio TKX. it has a 3.27 first gear which will give it 4.11 type off the line acceleration but a nice cruise rpm in OD. Silver Sport recommended this set up and it gives you that preferable 10.0 ratio off the line.

This thread is 10 years old. Back then you could get good parts like the US Gears Tom's sold. Tom was alive, his best spiders were still made, Timken bearings were common and usa made, prices were lower on everything.

10 years later the theory still applies, TKOs are gone and so are many people and those good parts.

aargh!!!!

"'Related Threads" strikes again!

Don't feel too bad. That spreadsheet is as useful today as it would have been 10 years ago. And once in a while, the related threads "bug" digs up an interesting thread. Maybe it's not a bug...

If you have a 4 speed...from your rear gears to 370 gears will feel like 75HP difference. I have a 5 speed Tremec 600 and just drove home from 370's to 411 rear gears. The car feels like it explodes anywhere near the peak BP 383 4,300rpm TQ band. 3rd gear pulls like 2nd. 4th gear pulls like 3rd . My timing is all in at 2900rpm and 3rd/4th are amazing with a push back in your seat with just a push of the throttle anywhere ...everywhere above 2800rpm. GEARS.