npbassman

I apologize in advance if this question has already been asked, but I suspect that the previous owner lowered the gears in my 93 coupe, ZF6 and I'd like to figure out what I have back there. I have RPO code GM3 (3.45's) but, I have two examples that make me doubt that. First, my wife followed me home one day and when we got home asked why the heck I was driving so slow. I had a temp plate on and I know I kept it at 60mph on the way home but shes aid it was a lot slower than that. And yesterday I was in the fast lane with the speedo clocking around 75-78mph. However, there were two little old ladies in the slow lane (no offense to the little old ladies of the world), one in a toyota minivan and the other in a Rav4, and they were in the slow lane keeping up with me with not problem at all. Makes me think that there's no way I'm doing almost 80.

That said, what is the easiest and most accurate way to tell what what I have back there? I'm thinking of having someone follow me at 60mph and I'll see what my speed is. Then I guess I have to do the math?

Mike

Aurora40

You could jack the car up, put it in neutral, and turn the rear tire. See how many turns it takes before the driveshaft rotates a full rotation. That will give you a ballpark idea.

If you want to compare your speed, just use a portable GPS device vs having someone follow you.

jhammons01

They etched it by hand on the housing when they put the gears in there. Clean off the case and look for etched writings. it'll tell you exactly what gears are inside from the factory

CFI-EFI

The stamped numbers on the diff is the factory installed ratio, as stated. If the gears have been changed INSIDE of the diff, it usually isn't noted on the outside of the diff.

Counting the wheel turns for a drive shaft revolution won't be possible unless you have overdrive gears (you don't) in your diff. Turn the wheels (both equally) exactly one turn and count the drive shaft revolutions. It won't give you just an approximation. Carefully done, it will give you the exact ratio. The relationship of the number of turns IS the definition of ratio.

RACE ON!!!

Aurora40

Ah right, of course. I was thinking about it wrong.

As for the exactness, I would think one might have a hard time telling a 4.09:1 ratio from a 4.10:1, for example, using this method. Or perhaps even a 3.45:1 from a 3.54:1. Though I can't speak from experience.

jhammons01

Ah, I re read it, that is true. I didn't consider that he was suspecting that they may have been changed. But damn...if He had 3:45s....someone changed it from there?

CFI-EFI

It may be difficult to tell a 4.09 from a 4.10, from a 4.11, but the number of turns IS what the ratio is all about. Carefully done, it is relatively easy to tell a 3.45 from a 3.54. In the 4.10 range, the difference is as hard to tell in the speedometer reading and the performance of the car as it is to tell in the counting the revolutions of the drive shaft.


From the opening post, the drive shaft has to be spinning too fast (lower gears) than the vehicle speed reflects. 4.10 gears in a D44 would do that for any stock ratio. Likewise 3.54 or 3.73 in a D36.

RACE ON!!!

Crepitus

You should be able to get pretty close by pluging your numbers into one of these tables. yhttp://www.keislerauto.com/speed_analyzers.php?sa=gm_5