libolt

Hi,

The previous owner of my 1991 Trans Am swapped out the rear end going from a 2.73 gear to 3.42. Now the drive and driven gears in the 700R4 transmission have not been replaced yet and the speedometer is inaccurate.

So I've searched the forums and see that a 15 tooth drive and 41 tooth driven gear ar recommended.

However I also went to: http://www.700r4.com/speedoCalc/speedocalc.shtml and entered my tire information (245/50 R16) and gear ratio and it came back with the 15 tooth drive gear but a 40 tooth driven gear. Now my tires are Kumho Ecsta 711 245/50 R16 tires as mentioned abve.

I'm trying to gather as much information as is possible on this because the mechanic I took the car to didn't know what was required to get the speedometer to read accurately.

So I would really like to know what drive and driven gears the transmission needs and if anything else needs to be done since it's the electronic/VSS speedo. Messing with the transmission is past my skill level. I just want to have the correct info for the shop that I take the car to.

Thanks

Mike

krisb410

iTrader: (9)

I did this swap a while back, here is what you need:

-15 tooth drive gear part # 8642620 list price $6.24
-41 tooth driven gear part # 25513048 list price $6.24

You would actually want a 40.5 tooth gear, but thats not gonna happen. Any way, that's all you have to do to get the speedo to read correctly.
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Do it yourself and save money. It's so easy, here's a link to directions with pics

http://sethirdgen.org/vss.htm

87zjeff

iTrader: (4)

i put a 4th gen rear into mine that had 3.42 and i used the 15t drive and 40t driven and it has been accurate. the speedo calculator at bowtie overdrives is the one i found to be the most accurate. it was the only one that matched my stock set-up(which i used to check the accuracy of the calculator). when i tried tci's and patc's calclators against my factory set-up they wouldn't match. btw, i'm running the exact brand and size tire that you are.

libolt

Sorry for resurrecting this old thread, but I am just now getting around to doing the speedometer gear conversion. I will be purchasing the 15 tooth drive gear and 41 tooth driven gear soon from bowtieoverdrives.com. However after doing more searching on the forum here I see that I may need to purchase a new 40-45 tooth sleeve housing as well. Is this correct since my car came with a 2.73 rear end originally, which according to the calculator would put the current drive/driven gears at 17/36?

I just want to make sure I have all the information possible before I go through with this.

Thanks,

Mike

dyed

You will need the sleeve for the larger gears. If you check one of my posts from a couple days ago, it has part numbers for it in an excel spreadsheet, along with the most accurate calculator I've found. All the others round some of the values, so I just used actual values for everything and don't round any of it. If you really want to check the thing, all the calculations can be followed from within the spreadsheet, or you can just ask. Keep in mind that you may be better off to use a 17t drive, check at least.

libolt

Dyed,

Thanks for the response. I tracked down your spreadsheet and used it to calculate. With the 17t drive gear I would need a 45.68 driven gear which is practically 46 teeth. So it seems I would be better to go with the 15t drive gear and 40 or 41t driven gear. What it calculated was 40.3t driven gear. In my searches on the forum I have read that a 40t driven gear will cause the speed to read slightly slower than what it actually is whereas the 41t gear will cause it to read slightly faster.

Right now I'm still leaning towards the 41t gear. Either way I'll be purchasing the 40-45t sleeve.

Thanks for all your help.

84redta

iTrader: (34)

not exacty sure how the tach is hooked up...but if it's slightly slower you'll rack up slightly less miles. Just a thought if it is hooked up how i think it is. I may be absolutely incorrect though.

krisb410

iTrader: (9)

libolt:

1991 cars use an electric vss, make sure you buy the gears for the vss and not a cable driven speedo. I reused the stock vss when I did my rear swap and the speedo is dead on.

libolt

krisb410:

That makes sense. Now I'm wondering if the gears on bowtieoverdrives.com are the correct ones for my car. They don't go by the GM part numbers that you listed in your last response.

These are the gears: http://www.bowtieoverdrives.com/cata....php?ITEMID=20 and http://www.bowtieoverdrives.com/cata....php?ITEMID=32

If not, I entered the part numbers you provided at gmpartsdirect.com and found the gears there. I won't need the sleeve because of the VSS right?

Sorry for all of the questions, this is all foreign territory for me.

Thanks again.

krisb410

iTrader: (9)

This one is the wrong gear http://www.bowtieoverdrives.com/cata....php?ITEMID=32

I bought mine at the local dealership. You could probably call bowtieoverdrives and see if the carry the vss gears, I'm sure they do.

libolt

krisb410

I went ahead and ordered the gears from gmpartsdirect.com using the part number you gave me in your original reply. Hopefully in the next week or two I'll be able to have them installed in the car and get the speedo reading more accurately.

Thanks for everyone's help.

krisb410

iTrader: (9)

They are real easy to install, here is a write up on how.

http://sethirdgen.org/vss.htm