If I change the rear gears from 2.73 to 3.23, should I also change the VSS? The Chevy parts guy said there are different VSS for each gear ratio.

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Chuck Rini
Perry Hall MD
91 Z28 305TPI Convertible Black
92 RS 305TBI T-Top White

 

Its not the VSS but the speedo gears that need to be changed. Depending on your current configuration (gears/tires) to your intended configuraation (gears/tires) will mean either changing one or both of the speedo gears. The drive gear is on the output shaft of the tranny and does involve some work to replace. The driven gear is what the VSS is connected to and is fairly easy to replace.

Also, do you have an automatic or a standard?

 

I need some help also I went from 2.73 to a 3.27 in my 91 RS which one or what do I need to do to fix my speedo?

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88 IROC 350 tpi, SLP cam, SLP headers 1 3/4, SLP siamised runners, Accel 58mm tb, Comp 1.6 RTR, Flowmaster 3" catback but not cat, TH350 with B & M Holeshot 3000 stall, 3.27 9 bolt posi disk rear, Vortech S-trim Supercharger, K & N and 24# SVO injectors

91 RS t-top's, tbi 305, 15.7 in 1/4,
x911 Kenwood cd player 2 12" Solo-barics L7's,RS 5.2 pack in kick panels, impulse 6x9's, a ZX460 and more to come soon.

 

3.27s would be the same as for 3.23s (the gears are not fine enough).

215/65/15 - 39 (Brown) Driven & 15 (Grey) Drive

245/50/16 - 44 (Dark Grey) Driven & 17 (Red) Drive.

Your stock drive gear is 17 with 2.73s (differing driven gears depending on tires), so if you were to select tires that give you the same overall height (25.5") as 245/50/16s, then you would only have to change the Driven gear which is the easy one to change.

 

These are all for automatics BTW. Manuals have completely different types of gears. For 3.08s they are 22 (Grey) Driven/9 (Red) Drive; 3.42s, 19 (White) Driven/7 (Purple) Drive. I am not sure if GM made other Drive and Driven gears for the manuals to allow you to select a combo that will correct your speedo for other rear gears.

 

But if the VSS is not changed, won't it send a weird reading to the ECM? I am not as concerned about the speedo, I alrady have a Cyberdyne unit that adjusts the speedo.

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Chuck Rini
Perry Hall MD
91 Z28 305TPI Convertible Black
92 RS 305TBI T-Top White

 

The VSS gets it reading from the speedo gears. If you change gear ratios and speedo gears, the VSS will read the same as it did before the swap.

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-Justin-
T-Top '86 5.0L LG4 700R4 WS6 Now Bilstein equipped!
T-Top '92 5.0L TBI 700R4
My '86 Firebird Homepage
The F-body Model Kit Pictoral Archive (updated 9/18/00)

There can be only one!!

 

I agree with Justin, the VSS is the same for all cars. It just sends a signal that corresponds to the number of revolutions that it counts from the number of times the driven speedo gear turns.

The ECM takes these signals and converts it into the speed for the ECM and electronic speedo. When the VSS turns faster, it has no way of knowing whether it is due to the car travelling a faster speed or because the tires or gears have been changed.

Thus, if you change your gears or tire size, the only way to correct it is either change the speedo gears to correct the number of times it turns the VSS or splice a Cyberbox (IIRC) into the circuit which electronically alters the signal.

 

Chuck, You do indeed need to change the VSS as well as the driven gear for the speedo to read correctly. Here is why. The new larger driven gear is larger diameter. It changes the centerline location for the VSS, forcing it to be further away from the drive gear. Make sense? The VSS you currently have is p/n 10456021 and has a red driven gear associated with it. The new VSS you need for the 3.32 gear ratio rear end with the A4 is p/n 10456089 and has a light gray driven gear asssociated with it. Sorry, but I can't find my receipt, or i would have the p/n for the driven gear as well. I recently swapped in a 3.45 gear rear end into my 90 RS and temporarily am using the closest VSS and driven gear available. I still have a small speedo error, but not as bad as if I hadn't changed it. I am planning to swap in a World Class T5 in the next couple of weeks. If you can wait I will have the VSS and driven gear you need available. E-mail me if you are interested. I hope this clears things up. Good luck.

 

Lonsal, are you sure that you are not referring to the sleeve?

 

Nope, I'm not referring to the sleeve. The 2.73 and 3.27 (or 3.23) use the same drive gear. So I didn't have to change the drive gear or sleeve. But the driven gear is larger diameter (because it has more teeth) and requires a different VSS as well. The center of the VSS gear shaft is not concentric to the housing seal diameter. Check it out with the Chevy Parts Dept.

 

Actually, there is a combo that uses the 15 tooth drive gear and with 3.73s you must switch from the 17 tooth drive gear to the 15 tooth drive gear to make it work. 3.23s with 215/65/15s used the 15/39 combo whereas the 3.23s with 245/50/16s used a 17/44 combo.

With my 3.23 to 3.73 conversion, I am only having to change the drive gear (17 to 15) so I am aware of the sleeve. Since my driven gear is the same (44 tooth) I was not confronted with any VSS change. Though I am putting in a new 44 driven gear, just because I have learnt (the hard way) that you should never match an old gear with a new gear, best to replace both for maxium longetivity.

I should also point out that for a better 3.73 combo 15/45 is a better match if you have tires smaller than 26" in diameter. I just never mention it, as the 45 tooth driven gear was never offered with VSS, only cable driven speedos, so I am not sure if GM makes such a combo for VSS.

 

Ok so in my switch from 2.73 to 3.27's all I need to do is switch the driven gear? If so ?I have a 700r4 from a 88 with 3.27's can I take that driven and use it in my 91?

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88 IROC 350 tpi, SLP cam, SLP headers 1 3/4, SLP siamised runners, Accel 58mm tb, Comp 1.6 RTR, Flowmaster 3" catback but not cat, TH350 with B & M Holeshot 3000 stall, 3.27 9 bolt posi disk rear, Vortech S-trim Supercharger, K & N and 24# SVO injectors

91 RS t-top's, tbi 305, 15.7 in 1/4,
x911 Kenwood cd player 2 12" Solo-barics L7's,RS 5.2 pack in kick panels, impulse 6x9's, a ZX460 and more to come soon.

 

Glenn, what is the best combo to use with an auto, 3.73's, and a 26.1" tire (255/50/16)?

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1989 IROC-Z 5.7L

 

44driven/15drive with 255/16/50s and 3.73s will cause your speedo to overrate your speed by 1.5%. 45driven/15drive with 255/16/50s and 3.73s will cause your speedo to underrate by 1.5%.

I found the 44 driven gear easier to get than a 45 driven gear and will allow you to install slightly larger tires and still be reasonably accurate (3% underrated with 275/50/16s with 3.73s). Typically, all speedos are 2-3% overrated with the stock recommended tires. With my 3.23s, 255s were only .5% underrated while with the stock 245s were 3% overrated at the speedo.

On my 3.73 setup, I am going with the 44/15 combo, which I hope to have installed sometime in January-February. I just have to make the time to swap the axles.