This is a 71 GS350 with a TH350 trans.. The speedometer reads 70 mph when the car is going 60 mph (14% error). Tires are 245/60-15 (26.6"). 3.08 axle I think. Drive gear has 18 teeth. Driven gear has 35 teeth. The speedometer housing reads "36 37 38 39." Can I install a gear with 39 teeth without changing the drive gear? Thanks.
Yes you can. But your desired correction is 70/60, or 1.1667. Multiplying that by 35 teeth gives 40.8 teeth. You ought to put in a 41 tooth driven gear, and change to a 40-44 housing. I once managed to rotate the housing some to cover a different gear. good luck, Bruce Roe
That drive gear should be fine. The plug holding the driven gear probably needs to change, to move the larger dia driven gear a little farther from the drive gear. First time I did that (in 1974), I just rotated the plug some, the hole is offset (cause I didn't know any better). Bruce Roe
Oh, and if that 70 at 60 statement is off, so are these calculations. You might want to double check it for accuracy. Bruce
I would recommend checking with this web site. There is a calculator place on it. But call and ask to talk to Kerry, he is very sharp on this subject. http://www.transmissioncenter.net/speedometer_calibration_______va.htm#350
Apparently the 41 tooth speedometer gear is quite popular since it is out of stock and/or discontinued from every supplier I could find. I am going to place a want ad, maybe I can find a used one. Thanks everyone for your help.
http://www.bgsoflex.com/speedo1.html According to the above calculator, you need a 42 tooth driven gear. Make sure you have a BOP tranny, it has a different output gear housing than a Chevy. Buick trannies have bigger housing with a horseshoe shaped retainer. From my personal experience, I would discuss with Kerry at PATC, I got many wrong answers from others, even at PATC. He is the man!
Forgot to ask if your existing gear housing is plastic? I needed a new one and got an aluminum one, much better. I broke the plastic one getting it out. It looks to me like you do need a different housing if switching from your current gear to a 41 or 42 anyway. Good luck.
Thanks Roger. I didn't have any trouble finding the 40-45 gear housing, $45 from AutoPartsTomorrow ebay seller. ACDelco aluminum part. The trans is for sure a BOP TH350. I know the empirical calculation says 42 teeth, but the tire rolling radius may not be exactly 26.6" as the book says. I measured the speedo error on the highway so I was going to go with that rather than the calculation. Also I would rather have the speedo read slightly high than slightly low. Thanks again.
Sounds like you got the situation in hand. Mentioned the 42 because it may be available and get you pretty close.
I may have to find an alternative route. TCI was buying gears from GM and selling them to JEGS, Race Mart, etc... TCI discontinued them because they were not competitive and has no stock left and GM no longer makes them. So maybe one will turn up on ebay or someone will sell me a used one. Cheers.
I got lucky, needed a 18 / 40 combination. Looks like 41,42.43 are all our of stock, maybe permanently! Still haven't put my new ones in yet, waiting for the tranny shop to have an opening. Not in the mood to remove the tail shaft housing myself. Needed to change the driver gear too. Currently running with no speedo, getting used to using the tach and no speeding tickets yet! I have a used 34 tooth driven gear and will have a used 13 tooth driver, maybe I should hang onto them.
There have been so many ads for speedo gears, its hard to believe they are no longer available. If 41 teeth is the problem, you could go 19 drive, 43 driven (43.102 is ideal). I know the TH350 can use the same driven gears as a TH400, did you check 400 gear availability? I have every 35 to 45 400 driven gear, I'm not sure what the range of TH350 drive gears is. Bruce Roe
Yea, 41 teeth is the closest. I checked the speedo again last night and made some calculations. With the 41 tooth gear the car would be going 70.3 mph when the speedo reads 70 mph. With the 40 tooth gear the car will be going 68.6 mph when the speedo reads 70 mph. So I could get a 40. I also learned the gear housing ("sleeve") is different between the TH400 and TH350. But the speedo driven gear is apparently the same.
I have some 41 tooth gears and I think I have some of the higher tooth housings still. If your interested I sell the gears for $14.95 and the new housings new I think were about $38.95, used $15.:beers2:
Bruce, thanks for sending me the 41 tooth gear! :beer I haven't installed it yet, that's next on the "to do ' list.
