trying to find out the final drive of a transmission. its out of the car and the guy says its a ATS 4.9 final drive. without taking the transmission apart to look at the final drive can i do this? put the tranny in 5th gear, mark the input shaft, stick an axel in the tranny spin it and count the rotations it takes the input shaft to do a 360 to see if its a 4.9? should come close to the mark if you can check it like this.

well hopefully someone can give me some insight on how to check it out taking the case apart. thanks

 

Yep, you got it; You'll need your 5th gear ratio though as well (depends what tranny you have). You'll need to do more than just one turn to tell the difference between 4.4, 4.785, 4.929 though.

The equation would be this:

(Final drive ratio) = (# turns input) / (# turns axles) / (5th gear ratio)


You need a bunch of turns; even with 100 turns of the input shaft the output will still be less than one turn different between an ATS and JDM f.d.