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I picked up a T5 this weekend. It's out of a stock 91 RS 5.0 with the 3.08 rear axle.
My car has a 3.42 rear axle.
I haven't looked inside to see what drive gear there is. I just wanted to see if anyone could verify that if I have the red, 9 tooth drive, I need a yellow 24 tooth driven or I can get the blue 7-tooth drive and a white 19-tooth driven.
Last edited by Tootie Pang; Jul 1, 2019 at 10:31 AM.
The 9 tooth won't work. It'll be the 7, and 18, 19 or 20. Which one will depend on your tire size.
MEASURE your tire height with the car sitting on level ground. Not the "height" really, but, the distance from the ground to the center of a rear wheel. DO NOT use the "trade size" on the sidewall to "calculate" the diameter, it will be WRONG. Always is. Somewhat close, but only approximate, at best. Measure both rear wheels and take the average.
Keep in mind that this is a little bit of a "measure with micrometer, mark with chalk, cut with axe" type of exercise. You're going to end up with a gear that has somewhere around 20 teeth, which is a 5% increment, more or less, from one to the next. That's as close as you can get... there will ALWAYS be some degree of error.
You can be VERY accurate in "calculating", if you want. Working backwards from the universal speedometer constant of 1000 turns per mile. all you have to do is figure out how far your tires turn in one revolution; divide 1 mile by that; multiply that by your rear gear (I prefer to use the actual tooth counts, 41 & 12 in your case, not "3.42"); divide the result by 1000; then multiply by your drive gear, which will be 7.
So: let's say you MEASURE your wheels, and discover that the center of the wheel is 13.25" off the ground. In that case, the tire rolls 13.25 x 2 x pi inches per rev, or about 83.25"; which is 6.938'. It will therefore roll out 761.06 times per mile(5280 divided by the 6.9xx number). Multiply by 41 and divide by 12 (your rear gears) and you will see that the drive shaft turns 2600.3 times per mile. Divide by 1000 and multiply by 7, which gives 18.2. Since you can't buy a gear with fractional teeth, you'd have to buy an 18 and accept that slight error... it will read about 1% high.
Perform the same set of calculations starting out with your MEASURED wheel radius - pull a tape measure on it, NOT "calculate" or "assume" or "look up" - and see which one you need.
No "calculator" you find on the Interwebz is going to get you any closer than doing it this way. In fact any one of those that's halfway decent will do the EXACT SAME series of steps.
Thanks very much everyone. So I do indeed have the (7T) red drive and a (22T) gray driven for that 91 RS (pics). I'll pick up a (9T) blue drive and (19T) white driven and fiddle away! The clutch was completely burned apart and the trans feels smooth and tight. Fingers crossed.
I like going by the service info from GM. Start with 7/19, fiddle with 7/20 (what John really meant to say) if it reads too fast.
Not the first time I was was expecting to go forward & put it into into "reverse" instead !!! I should have just posted the 87 Gear chart and shut up. Chart shows gearing for T5 with 9-Bolt 3.45(6?) so just follow that for a guide based on tire size.
for 3.42 91 service manual calls for a 7 tooth PURPLE DRIVE and a 19 tooth WHITE DRIVEN when on stock 15" wheels/tires. 87 service manual calls for a 7 tooth RED DRIVE and a 19 tooth WHITE DRIVEN when on stock 15" wheels/tires. 87 service manual calls for a 7 tooth RED DRIVE and a 20 tooth BLUE DRIVEN when on stock 14" wheels/tires.
235/60/15" tire is 26.1" 245/50/16" tire is 25.6" 205/70/14" tire is 25.3"
Chart shows gearing for T5 with 9-Bolt 3.45(6?) so just follow that for a guide based on tire size.