i have a 1991 camaro with the tbi 305 and t5 trans now some one installed 3.42 gears in the rear end and now my speedo 10 to fast i found out i need a 27 tooth gear but i cant find one anywhere! been at parts stores and having trouble finding them online so does anyone no were i can get one?

 

https://www.thirdgen.org/forums/tran...gital-sgi.html

 

There is no such thing. Which of course just adds to the trouble of finding one.

T-5 driven gears are in the range of 19 to 22. Drive gears are either 9 or 7 teeth.

This being the case, the calibration is EXTREMELY coarse. That is, you're not usually going to ever get "perfect", because the "steps" are so large. You get the right drive gear, basically 9 for 2 series rear gears or 7 for 3 series, then the closest driven gear possible to "ideal" for your gears & tires.

But there's one more wrinkle: a given # driven gear for 9-tooth drive gears, is different from the same # driven gear for 7-tooth drive. You need to swap them as a pair.

In the case of 3.42s and the usual size tires for these cars, that would be a blue 7-tooth drive, and a red 21-tooth driven, if memory serves. If your tires are on the large side, like over 27" ACTUAL MEASURED diameter - NOT "calculated" from the "trade size" printed on the sidewall - 20 tooth might come out closer.

 

T5 driven gear range for cable drive
Speedo Gears Part NO.
18 Brown 3987918
19 White 3987919
20 Blue 3987920
21 Red 3987921
22 Gray 3987922

7 Purple 14071731
9 Red 14053347


To calculate the driven gear use (# Drive Teeth X Axle Ratio X Tire Rev. per Mile) /1001

So (7 x (20168/tire Diameter) x 3.42) / 1001

 

The math works very well. Mine is a half a mph slow at 60, which is nice, because when I bought the car, it was 15 mph slow.


It might be easier if we knew what the axle ratio was before it was changed, and if the speedometer was accurate then...

 

If you can find a T5 from a '90 - '92 3.1l Camaro or Firebird, it will have the proper speedometer gears. Those cars are factory with 3.42 rear gears.

RBob.