Ok, I'm usually just a lurker around here but I need some help. I swapped my rear gears from a 2.73 to a 3.73 and I am trying to fix the speedometer in a 700-R4. I know what I am supposed to need, a 15 tooth drive gear and a 44 tooth driven gear (I had to get one for an '87 because it wasn't offered in '88), and a VSS part number 25007339, which was changed/updated to 25007308. I got all that but the new driven gear won't fit. It's about 1/8 inch larger in diameter than my old 38 tooth one. I thought that's why I needed the different sensor for the different size gear but it looks exactly the same as my old one, I looked at others at the dealer just to be sure it was not misboxed and it is the right part, so is are the drive and driven gears. Nobody else can seem to tell me what the **** is going on so I would really appreciate any help. :hail:

 

As far as I know, all the gears can be used together. AKA the same size....

I never had one that was 'bigger'

They must've gave you the wrong one or something...

 

This is just a wild a$$ guess, but you may need to change the drive gear to get the driven gear spinning at the correct rpm.

And yes the driven gears will be different diameters.

I'm not understanding why you would need a different vehicle speed sensor.
Your speedometer is calibrated to indicate (if i remember correctly) 50mph at 1000 cable rpm. The speedo and vss don't care what ratio the rear end is.

Here is another IIRC, to replace the drive gear, all you need to do is pull the tail shaft housing, remove a spring clip, slide the old drive off, slide the new one on and re-assemble everything. I think GM used an o-ring to seal the housing to the trans.

I can't complete the post without my standard saying:
Get a Year Specific Factory Service Manual.

 

I just changed my speedo gears last night on my 92 RS cause I put 3.73's in. I didn't have to change the VSS, everything fit together fine. The dealer probably made a mistake.

One more thing, did the stock 87-88 have a VSS on the tranny? My old 87 was cable driven, so I needed a new speedo gear sleave for the driven gear. The part number I think is different for the cable driven speedo gear as well.

 

I've already replaced the drive gear with a 15 tooth one. That was no problem.

Apparently they started using the electronic speedo in '86, but I guess it was only on some cars.

The computer at the dealer lists a different vss for 35-39 tooth gears and 40-45 tooth gears. I thought it to compensate for the different dia. but now I don't know.

My mom has a '91 firebird, so I went and pulled her vss just for the hell of it and it looked totally different from mine, and used a 39 tooth gear that was the same dia. as my 38 tooth. No clues there.

Where can I get a year specific factory service manual? About how much does one cost? Will it tell me anything different than the dealerships computer?

 

I've seen manuals on ebay for 20-30 bucks.
You may want to check the Helms website for a brand new one.

I thought they didn't use the electronic senders till 90 or 91.

Sorry I'm not any more help.

 

The 84 Berlinetta was the first F-body to ever recieve the electronic Speedo sending unit.....

Then Firebirds then began to use then around 86 or so & the "regular" Camaro line-up finally got the electric sending unit due to the the new interior in 90.......

I have seen the different sized "Driven gears" and that just seems to be no problem. ( a 39 tooth is smaller than a 45 tooth )

I also think that by using the "Sleeve" that was mentioned by someone here earlier you can interchange the non-electric & electric speedo gears. I have not had to use a sleeve yet !