Ok before I get flamed I did look at the sticky's and the one guy who asked the question I'm looking for the answer to, never got answered.....

I recently swapped rear gearing to 3.73 in my Vert. Long story short I'm trying to get the speedo working accurately. The transmission shop said I have a 17 tooth drive gear and would need a 47 driven gear (which isn't made... I know). I figured based on the calculator in the sticky I can replace the 17 tooth drive gear with a 15 tooth and then get a 42 tooth driven gear and it will be within .5% accuracy.....

Now for the question. Can you mix and match any of the drive and driven gears? Are there combinations that won't work? Or more specifically can I do a 15 and 42 and not have it destroy the gears because they won't play nicely?

 

Bump...... anyone? can i use a 15 and 42 tooth, or is this not a valid combo?

 

It's a valid combo and would have been used in 3.42 or 3.45 geared third gens with 215/65R15 tires. I'm assuming for 3.73 compatibility your tires are taller diameter. (All the calculators you'll find online differ from what GM used in production vehicles.)

Across the usual third gen transmissions, the o.d. of the drive gear matters when determining compatibility of the driven gears. There's also the depth of the driven gear (such as different driven gear sleeves for different driven gear tooth ranges in the 700-R4 and TH400, to name two.) The loose meshing, greater diameter of the drive gear and quiet material allow a single drive gear to be compatible with the broad range of driven gears.

Thanks for doing your research.

 

Thanks, yeah i'm running 245/35/ZR20's according to the excel sheet it's only about 1" in diameter larger than the stock 245/55/R16's, but by my calculations it's really 2.5".

Thanks again for the confirmation, i'll be ordering them this tonight.

 

Well I'm back. got the gears (that 15 tooth Drive gear was a challenge but finally found it at TCI-Auto, and as of right now i don't have anything real positive to say about GMPartsDirect.com as i am STILL waiting for them to ship my two 42 tooth gears i ordered on the 10th. to top it off you can't cancel the order, and i already got one 42 tooth gear through a local dealership).... anyway back to topic. took it to have the gears installed turns out my speedo died because the VSS failed. ok fine.... they put a new VSS in it, setup the 15 and 42 tooth gears put it back together and ran it on the lift then at 40MPH the VSS breaks again and the 42 tooth gets ground down to nothing. supposedly the VSS felt stiff so it was suspect it was bad.

not that i want to be insulting but is it a definite positive these gears play nicely together? i want to make sure before i keep dumping money into gears that i'm just going to keep grinding to the ground.


Edit: Figured..... right after i hit reply on this message i get my tracking number for the two gears from GMPartsDirect.com....

 

The driven gears all have diferent outer diameters. This is why there are different carriers, rather than just a small bore in the tailhousing. If you try a 34-tooth gear in the carrier for a 45-tooth, the gears won't mesh.
In the early years, there was an aluminum carrier for 34-39, and another for 40-45. But the last time I ordered an early-style 45-tooth gear, it came with it's own dedicated plastic carrier.
However, all the gears have the same tooth cuts, as far as angles and depths.
If you keep destroying speedometer parts, I'd suspect you have some hidden problem with your tailhousing. Maybe it was machined at the limit of tolerances? If so, it's not that costly to try a different one.

 

also make sure your carrier for the driven gear is clocked right. the hole is off center just a little bit, and if you don't pay attention like me you kill the gears.

 

appreciate all the responses. the shop i took it to put the first VSS in the car with the 15/42 combo and chewed the 42 gear and killed the VSS. said normally it chews both gears so they wanted to try again.
2nd attempt - New VSS and a new 42 gear - a block down the road speedo died... but gears were fine.
third attempt - New VSS - died, but gears fine.
fourth attempt - New VSS - FINALLY worked and has been working since. i got the car back around the first of march, a week later i put my wonder-bar in and discovered my tires were shot, replaced them and discovered my steering parts are bad.... but at least the speedo still works.

overall the speedo works great now, and only reads 1MPH faster then i am actually going. at least calculated compared to a Garmin which tells me my speed. so for now all is well.... hopefully it won't break again.

 

it's good the speedo is at least working. that is a lot of attempts good thing the gears are not gold, sounds like you have my luck 1 step forward 2 or 3 back.

 

Indeed sir - i have to say my luck has always been if something can happen it will and it will do so in quick enough succession to never allow me to save any money for things i actually WANT to do.... i.e LQ9/T56, new top, etc