Changing rear end gear ratio

[Jeff C.]

Can anyone comment on the degree of difficulty of installing and setting up new ring and pinion gears in a 67 BB? Would like to change from a 3:08 to a 3:70. I do most of my own work but have never done this. I plan on buying new gears and a installation kit. any comments or advice will be appreciated.

Jeff

[Wayne W.]

Re: Changing rear end gear ratio

Its cheaper to buy a good used one and keep your other in case you want to put it back someday.

[Bob Booth #33372]

Re: Changing rear end gear ratio

Jeff,

I also plan to change the ratio in my '72 from a 3:08 to a 3:70. I did some research and reading (I also do most of my own work) and came to the conclusion that I will leave this chore to a professional. I priced the ring & pinion and rebuild kit out at about $325.-$375. (depending on shipping charges) and found a reputable person to do the labor for $100.-$150. So, for about $500. I'm all set, no hassle.

As Wayne mentioned, as an alternative, you may want to find a used unit and keep yours intact for originality's sake. But as far as doing the work yourself, you may be more confident about your skills than I am of mine. Personally, I would rather spend the hundred or so bucks and not have to worry about it.

Good luck, Bob Booth

[Joe L.]

Re: Changing rear end gear ratio

Jeff----

I definitely wouldn't recommend tackling this job yourself. You might get away with it OR YOU MIGHT NOT. Then, you'll be yanking it out again and going through the effort all over. Or, you'll decide to take it to someone else, at that point. In my opinion, doing this job right the first time requires experience, especially if a more-or-less QUIET unit is important to you.

Having said all of this, I strongly recommend Wayne Womble's suggestion. Leave your carrier assembly as-is and purchase a complete 3.70 carrier. They are not that difficult to find. It might cost a little more, but you'll end up with TWO complete carriers and your original will be intact. Who knows, maybe you won't like the 3.70 gears? Remember, gas prices are going up.

[Jack H.]

Re: Changing rear end gear ratio

Easy job, IF you've got the tools and have done it seven times before! To get old parts out of pumpkin and new one's in, you need a special tool to stretch the case (without frature/breaking). Then, when the new parts go in, there's individual shimming operation that's iterative to get the setup dimensions correct. You'll be staring at 1-2 mil alignment issues here/there and need to know how to mark/read a correct gear mesh/preload pattern.

On the other hand, buying used pumpkin (without prior knowledge of what to look for in terms of integrity) can backfire on you too. Since many of these parts (case casting, ring/pinion gear sets) are essentially discontinued, you have to question what you're getting in terms of a 'bargain' in a used rear end!!!! I'd tell you NOT to save pennies by doing it yourself, go to a pro with experience, fork out the dough, BUT make him teach you as he works for you so the general knowledge is transferred in the process.

[Jeff M.]

Re: Changing rear end gear ratio

jeff I also changed my 3.08 for a 3.70. It will be the best money you ever spent for the performance you get. You will be surpised at how quick you get to 6,000 rpm. I would only have someone good do your rear end. I would also only buy a GM GEAR, yes you will pay more, but nothing after market will be as quiet. I got an NOS 3.70 gear from paragon($400). The 3.70 is the most popular and very hard to find. A richmond gear will cost you $268 from summit. SPEN THE EXTRA MONEY, you will be happy in the long run. I bought the rebuild kit and posi clutches from zip products for $200. I was also told the job would be $200 to install. I t turned out to be $400. Do not be surprised if your final price is higher than the $200 quoted, it is alot of work

[Wayne M.]

Re: Changing rear end gear ratio

Jeff --- Assuming you have a manual transmission, one other minor but possibly annoying point; your trans tail shaft probably now has the brown 18-tooth drive gear which is among the 4 the larger series of driven (various colored plastic) gears that transmit to the speedometer. Switching to the differential 3.70 gears requires a green 22 tooth plastic gear which is smaller in diameter, and this will not mesh with the gear on the trans tailshaft. So either you leave your current plastic sending gear in place and correct indicated speed and mileage downward by 20 %, or install a red gear (21 teeth) to reduce the error to 4 %. If this concerns you, check the archives, as there is a ton of discussion on this. I believe Joe Lucia had located a relatively unknown part # for a driven gear for 3.70 ratio that meshed with the drive gear for the 3.08 - 3.55 family.

I vote that you buy another pumpkin with 3.70 gears; keep the 3.08's for a rainy day.

[jeff chester]

Re: Changing rear end gear ratio

How much of a difference will the 4:11 or 4:56 make. My 68 427/390+ will only be a sunday driver. Plus I'm still a young pup..25.. it has 3:08, I have two rears, both 3:08's, darn. what about all these mail in catolog switch rear end places...? thanks jc

[Bob Booth #33372]

Re: Wayne, where would I find driven gear...

...(red 21 tooth ) for the trans. tail shaft you mentioned to correct speedo for 3:08 to 3:70 gear change (I have a 4 sp.)? I am also making this change to my '72. Is this a GM item only or is it avail. through a place like Ecklers etc.?

Thanks, Bob Booth

[Wayne M.]

Re: Wayne, where would I find driven gear...

Bob --- The red gear will only get you close (reading 4% faster than true). It used to be GM part # 3860344 (only GM makes them, to my knowledge). But the availability, and whether the part # has upchanged is in the archives. I searched under "speedo" and found the headers on 15-17 Aug ('99); 7-9 Sept; 6 Oct; but they wouldn't respond when clicked on. Read all the headers by Joe Lucia, at least. Now your '72 might be more complicated, depending on what engine/trans/rear ratio combo you now have. By then, GM may have simplified inventory for 4-speed transmissions, by only providing a single design of sending gear (on tailshaft) capable of meshing directly with only the 4 plastic gears covering the 3.08 to 3.55 family. Then, if you ordered a 3.70 or 4.11, you got an adapter that screwed on to the fitting on the trans tailshaft (where the plastic gear is inserted) that reduced the cable speed by a certain factor (say 0.833) which would allow you to use one of the brown, natural, blue, or red driven gears, along with the higher numerical rear end differentail ratio, yet still get a relatively accurate reading on speed and distance.

Try these archives dates, maybe they'll work for you.

[Joe L.]

Re: Wayne, where would I find driven gear...

Wayne and Bob----

Yes, for 1971 through 1974 Muncies there was only 1 speedomter drive gear(the one mounted on the transmission mainshaft)used. This is GM #3978758 which is a steel gear and still available the last time I checked.

This gear will mesh only with the "39879XX" series of driven gears. There are 5 gears in this series as shown below. The number of teeth is the same as the last two digits of the part number:

3987918-----brown

3987919-----natural

3987920-----blue

3987921-----red

3987922-----silver

According to the information which I have, for 1972 the "lowest" (mumerically) gear ratio which was available with a 4 speed was 3.08:1. This ratio required the GM #3987920 driven gear, so the 3987918 and 3987919 gears were not used for any 1972 4 speed applications.

Also, according to my information, the GM #3987922 (silver) gear was used with both the 3.55:1 and the 3.70:1 rear gear ratios WITHOUT an adapter. Obviously, there must have been some compromise in the speedometer accuracy with one (or both) of the two ratios if the same speedometer gearset was used. However, this may have been within acceptable production limits if, indeed, the information which I have is accurate.

According to my information, the only application in which a speedometer adapter was used for a 4 speed-equipped 72 was the 4.11:1 rear gear ratio. This ratio used speedometer driven gear GM #3987921(red) with speedomter adapter GM #6477965 which is currently available under GM #1565208.

It is very possible, though, that an adapter was actually used with the 3.70:1 rear gear ratio for 1972. For this 1971-74 period, the GM published information is "sketchy" and often inconsistent with reality. The only way to know for sure is to install the prescribed gear and have the speedometer checked for accuracy. If necessary, then, add the appropriate adapter.

[Terry M.]

Re: 3:70 ratio

Joe,

My 1970 with 3:70 gears has NO speedometer gear adapter box, just straight gear inside the trans. I don't know why 1972 would be any different.

Terry

[Joe L.]

Re: 3:70 ratio

Terry-----

Yes, your 1970 could be different. Here's why: 1970 was the last year for the 66-70 style Muncie with the 26 spline output shaft. This shaft had two different speedometer drive gears which could have been installed, one of which mated with the "small diameter" 39879XX series of driven gears (originally GM #3860341,3860342,3860342,3860344) which were also the only driven gears used in 1971-74. The speedometer DRIVE gear used for 1968-70 for use with these 39879XX DRIVEN gears was GM #3924098 (later GM #6261794). This is a GREEN nylon DRIVE gear. In its GM #6261794 form, it is still available. This nylon gear was used for 1968-70 only and is secured to the shaft with a metal clip, GM #1373122(but you won't find that one in a Corvette parts book, but that's the part; still available, too)

For 1968-70 there was another nylon speedometer DRIVE gear used. This drive gear was used in conjunction with the "larger diameter" speedometer DRIVEN gears, the GM #3860345, 3860346, 3860347, and 3860348 gears, which are still available under those numbers. This second DRIVE gear, originally GM #3924097 and later GM #6261795 was a BLUE gear and has been discontinued for quite some time.

The 3860345-48 series DRIVEN gears, mated with the 3924097/6261795 DRIVE gears, were generally used for the higher(numerically)rear axle ratios. Your 1970 with M-21 uses the 3924097/6261795 DRIVE gear in conjunction with the 3860345 DRIVEN gear which is a GREEN, 22 tooth gear.

For 1972 there was no equivalent of the 3924087/6261795 DRIVE gear produced. The only DRIVE gear ever available for 1971-74 Muncies was the GM #3979758 steel gear which is equivalent in tooth configuration to the 68-70 GM #3924098/6261794 nylon DRIVE gears. So, only the "small diameter" 39879XX-series of gears will work. HOWEVER, for 1971 a new speedometer DRIVEN gear was available which had not been available previously. This is the GM #3987922 SILVER gear of 22 tooth configuration. This gear has the same tooth count as the "large diameter" driven gears, but can be used with the 3924098/6261794 and the 3979758 DRIVE gears. This is the combination which I mentioned in my previous post is applicable to 1972s with 4 speed and 3.70:1 rear gear ratio.

For 1970 Corvettes with M-21 4 speed transmission, only 3.08 and 3.36 ratios used an adapter. For M-20 transmissions, 3.55, 3.70, 4.11, and 4.56 required an adapter(although I don't think that any of the latter 2 rear ratios were actually built with the M-20 trans).

So, that's the story. Simple, isn't it?

[Terry M.]

Re: Simple? Not

Joe you will never cease to amaze me.

Uncle!! and I don't mean that for Uncle Joe, but you can take it that way too if you want.

BTW: The 4:56 gears use the adapter box also, but I am not sure of their availibility in 1971-1972.

Terry

[Tom B.]

Re: Changing rear end gear ratio

JC,

A 4:11 or 4:56 will make alot of difference. If you're just driving on Sunday AND not out on the highway for long periods of time AND you really want one, then do it. But keep your other ratio / rear gears in it's housing.

PS, what's going on with your 68? Have you got it done yet? TBarr #24014