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Old 09-27-2007, 08:05 AM   #1
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Car: 1988 IROC-Z camaro
Engine: 355 TPI
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choosing the right speedometer gear

Im going to start a new post on this instead of building off an older post of mine. My car started life having a 2.73 posi and now it has a 3.42 posi. My question is do we know what drive gear it has for the speedometer and if so is there a corresponding driven gear that will allow my speedometer to be accurate again or do i have to swap both the drive and driven gear to get my speedometer to read correctly?
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Old 10-04-2007, 09:22 PM   #2
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Re: choosing the right speedometer gear

no one has an answer for this?
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Old 10-04-2007, 09:50 PM   #3
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Re: choosing the right speedometer gear

17tooth drive gear
45 tooth driven gear
provided you have a 26" tire

you will have to change both

http://www.transmissioncenter.net/sp...n_______va.htm
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Old 10-04-2007, 09:58 PM   #4
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Re: choosing the right speedometer gear

Thanks. Thats what i thought i would have to do but i wasnt sure. Ok I'll probably just put that off 'till after winter. Sounds like a nice spring project.
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Old 07-20-2008, 03:37 PM   #5
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Re: choosing the right speedometer gear

Well i have put this project off for far too long and now i intend on doing it. I have a few questions before i start though and order my parts. Does anyone know if i need a new Driven gear housing? From what i could tell on the website linked above it seems that the housing i have will be fine (used to have 2.73 gears). The second question is can i do all this fairly easily with the transmission in the car? Also, should i get anything else while im at it like the spring retainer clip and stuff that they show on this webesite?
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Old 07-20-2008, 05:38 PM   #6
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Re: choosing the right speedometer gear

I don't know what year GM changed, but my 91 has the electronic "Speedometer Drive Sensor". There is no speedometer cable, just a two-wire connector.

I spent a lot of time researching this issue both on this forum and around the net.

Everything I've seen indicates that the 2.73 to 3.42 swap requires a new driven gear (EDIT: red-stock, lt blue-new) and a new mechanical housing/electronic sensor (depending on year of transmission). The drive gear, if red, doesn't change.

I just upgraded to a 3.42 posi from a 2.73 open. I needed a new 45 tooth driven gear (EDIT: Lt Blue) and a new sensor. Note that the driven gear is a different style on the mechanical vs. electronic set up. The mechanical gear has a shaft integrated with the gear. The electronic version is just the gear and looks like a hat.

First, get under the car and find out if you have a mechanical or electronic speedo setup. You can visually check it easily. Then, remove the one bolt holding it in and pull it out (some oil will drain, but not much). Check the color of the drive and driven gears.

I'm guessing both will be red, but the driven gear might be blue since you have an IROC and it might have come with 245/50R16 tires.

Along with the link above, I found some good info here:

http://www.3rdgenformula.com/88/88speedometer_gears.htm

Last edited by Trick414; 07-20-2008 at 06:11 PM.
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Old 07-20-2008, 05:56 PM   #7
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Re: choosing the right speedometer gear

what about the ratio adapter, i figured this would be easier, even if its more expensive

i been told you can replace just the driven gear and get away with an accurate reading.....

im going 2.73 to 3.23 ....... what colors would i need, or what % ratio adapter .... i find the formulas to be confusing
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Old 07-20-2008, 08:03 PM   #8
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Re: choosing the right speedometer gear

From the research i did here it seems that i would need to change out both.
http://www.transmissioncenter.net/sp...n_______va.htm I guess im just going to crawl under it and figure out what needs to be done.
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Old 07-20-2008, 08:31 PM   #9
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Re: choosing the right speedometer gear

Quote:
Originally Posted by subroc View Post
From the research i did here it seems that i would need to change out both.
Both? Do you mean both gears? Or, the driven gear and the housing?

Yeah, get under there and figure out what you have and go from there. Speculation is tough.

I did the same thing until I crawled under it and found out what I actually have.
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Old 08-07-2008, 12:15 AM   #10
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Re: choosing the right speedometer gear

This is a great thread. I am doing the same swap with practically the same car/setup. Any info you guys find out is greatly appreciated. Ill put in what ever I find out also.
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Old 08-08-2008, 08:05 PM   #11
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Re: choosing the right speedometer gear

Im throwing a wrench into the mix for myself now. Im swapping out the 3.42s for a 3.27 9 bolt posi disk rear. The gearing is just too steep for the highways. Now i gotta get the correct gearing for that rear
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Old 10-21-2008, 06:44 PM   #12
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Re: choosing the right speedometer gear

Here's where I"m at:

Trans came from a car with 3.23's. Never taken it apart.

My car has 3:73's with the correct driven gear, and read accurate +/- 5mph at all speeds up to 100mph.

I am swapping a 3.42 rear in. Will I need to change the driven gear/Drive gear in the trans?
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Old 10-21-2008, 06:44 PM
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1986, 323, 373, 82, camaro, choosing, firebird, gear, gears, iroc, put, rearend, spedometer, speedo, speedometer, t56, trans
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