[davidgto]

I know this has been discussed but I have been unsuccessful finding it in the search prompt. Where is a good sight for determining the correct tranny speedo gear based on tire and rim size for a Muncie 4 speed in a 65 GTO.

[johns]

wallaceracing.com/

[Rich-Tripower]

Good info here.

https://www.chevelles.com/threads/se...#post-11389346

[davidgto]

Thanks for your help, fellows

[shermanator2]

1001 is the magic number that you need to know. That is how many times that you want your speedometer cable to turn every mile. Look up your tires on tirerack.com or elsewhere to determine how many revolutions/mile they turn. Multiply by your rear end ratio to get the number of revolutions/mile that your driveshaft turns. Divide this by 1001 to get the optimum speedometer gear ratio. Muncie car transmissions have either 6 or 8 teeth on the drive gear on the output shaft inside the tail shaft housing. You need to know which of these you have and multiply that by the ratio calculated earlier and that gives you how many teeth you need on the plastic driven gear.

for example, tirerack says my GTO's tires turn 786 revolutions/mile. I have 3.55:1 gears and 8 teeth on my drive gear. 786 X 3.55 = 2790 drive shaft revolutions/mile. 2790/1001 = 2.7875. 2.7875 X 8 = 22.3.

22 teeth was the largest driven gear that they made, so I lucked out. If my tires were shorter or my gearing lower, I would have to dissemble the transmission and change the gear on the output shaft to a 6 tooth gear.

and BTW, the plastic driven gears for the 6 tooth drive gears are not the same as for the 8 tooth. You need to get the matching ones.

[Gary H]

TCI transmissions has a very good calculator and tips on their site. They also sell both the drive and driven gears.

[Andre]

On my last change out, I didn't use the calculator as I was not 100% sure that the gears I had were 3.55. Using the Speedo app on my phone, I drove at 60mph on my speedometer for 1 mile and observed the speed on the app which was 54. I then drove for one mile at 70mph on the speedometer and observed the speed on the app. I forget what that was, but after some quick math, I realized my speedometer was reading approx 11% fast. I pulled out the speedometer gear (which was tan), and replaced it with a red one whose teeth most closely matched the % change I needed. In some additional research, I found that GM speedometers of the era typically read about 3% faster than actual speed.

[Tom Vaught]

Thanks Gary H for the link to the "drive" and "driven" speedometer gears.

Tom V.