[Jon Dailey]

I have a '79 T/A with a turbo 350 tranny. I just swapped in 3.42 gears in place of the 2.41s. I took it for a test drive and was cruising along at about 35 mph, I looked down, and my speedo said I was going 60. It is not even close. Can I just change the speedo gear that drives the cable, or do I have to change the internal gear on the output shaft too? Does anyone have any suggestions on what gear to use? If so, does anyone have one they would sell? Thanks for any help.

[Jon Dailey]

I have a '79 T/A with a turbo 350 tranny. I just swapped in 3.42 gears in place of the 2.41s. I took it for a test drive and was cruising along at about 35 mph, I looked down, and my speedo said I was going 60. It is not even close. Can I just change the speedo gear that drives the cable, or do I have to change the internal gear on the output shaft too? Does anyone have any suggestions on what gear to use? If so, does anyone have one they would sell? Thanks for any help.

[rocky]

I pickup a speedo gear for my T-10 from GM .I dont know if they have auto speedo gears or not but they had the 4-sp on the shelf.My car also has 3.42 gear good luck

[This message has been edited by rocky (edited 10-30-2001).]

[stu]

You will need to change both. There is a list of drive and driven gears for these transmissions, and one way to calculate which ones to use is to make the new speedo gear ratio equal to the old speedo gear ratio multiplied by the ratio of the rear gears. Sounds confusing, but isn't too hard. For instance, say the old one is 10:20 (drive/driven). Old axle = 2.41, new axle = 3.42. New speedo ratio = 10:20 X 2.41/3.42 =0.352. Would require a driven gear of 28.4 which is close but in practice you will find that there is a combination of drive/driven gears that are exactly right. Probably there is better way but I did this not knowing it.

[This message has been edited by stu (edited 10-30-2001).]

[455Dave]

Here's a couple of links to check out:

http://www.tciauto.com/tech_info/speedo_gears.htm

http://www.geocities.com/MotorCity/S...432/speedo.htm

http://www.lastgen.com/trannyinfo.htm


IIRC, the TH350 drive gear is held in by a clip an may be able to be changed out with the tranny in the car.

ST-10 owners will have a harder time since the steel drive gear is pressed to the shaft and the tailhousing can't be removed without dropping the reverse shaft thrust washers into the bottom of the case if they are not supported. Probably easier to pull the tranny if the drive gear has to be changed.

You might try pulling the driven gear and get a look at the colors of the drive and driven gears. Use the formulas above and on the web pages to see if you can match up a driven gear with the existing drive gear. It may not work, but it's worth a shot.

Dave


[This message has been edited by 455Dave (edited 10-30-2001).]

[TINFY Racing]

you can change the internal drive gear on a th350 with out dropping the tranny. i had mine done by and friend. if you have factory exhaust it will get in the way a little but all you have to do is remove the tail shaft cover to access the gear. you may want to make sure the car is lifted in the back so that most of the tranny fluid stays in the tranny. also go ahead and replace the tail shaft seal while your under there. Trey

[Skip Fix]

An old guideline that I read years ago was drive the car at 55 and use 1 tooth per 5mph change on the driven gear(go bigger if shows too fast to slow speedo down). Easy to change out in the driveway. There are two different housings for the driven gear depending on the size of the gear, don't remember the break off hand.