Thanks to several forum members I successfully changed out my 2.92 open for a 3.42 posi in my 71 Riv ! Now the speedo is off significantly. Can anyone tell me what is the correct gear i need for the speedo to read correctly (color-teeth) I understand the housing is different as well. Thanks, Dan.
To choose the correct speedo driven gear, we'll need to know how many teeth are on your speedometer drive gear (the one on the output shaft inside the housing) and rear tire height. Devon
I am running 255-70-15s on the rear. I dont know yet what is in there (I havent opened it yet) But it did have a 2.92 stock for the rear ratio.
There is more to it than that. It depends on the speedo DRIVE gear. They are not all the same. There are 16, 17, and 18 tooth gears in most Buick THM 400s. Once you know the tooth count, rear tire height, and gear ratio, calculating the correct driven gear is simple. His tires are P255/70R-15's. They are 29" tall. ---------- Post added at 07:49 PM ---------- Previous post was at 07:38 PM ---------- In some of the Buick Chassis manuals, they list the DRIVE gear tooth count in the transmission section. If you have the original transmission in the car, you may be able to look it up that way. The original transmission for a 71 Riviera was a 71 BT. look at the tag riveted to the passenger side of the case. ---------- Post added at 08:04 PM ---------- Previous post was at 07:49 PM ---------- OK, I looked it up for you. The 1971 BT transmission had an 18 tooth gear. If in fact, that is the transmission in your car, then for a 3.42 gear, and 29" tire, you need a 43 tooth (purple) DRIVEN gear, GM part number 1362196. For the 2.93 gears, all else being equal, there should be a Red 37 tooth DRIVEN gear in there now. You will need another sleeve to run the 43 tooth gear. The sleeve you have now takes gears 36-39. You need the sleeve that takes gears 40-45. GM part number 1362293.
