Have never located the stamp on my axle housing tube. Have never located my cars build sheet.

So tachometer calculations have been my only verification method so far. With poor results, originally figured my car was a 3.08 but was working with a speedometer that is inaccurate and tires that were not factory height.

Then track performance helped me whittle it down to 2.78 but rpm readings where possibly too loosely observed at given speeds to be accurate.

All the recent gear talk got me curios. So i took a long hard look at my performance notes and voila !

The perfect pass to dissect materialized. September 30th 2018 on a 72 degree day the 69 ran a 16.003 @ 87.02 MPH she was sporting 245 60 15 BF Goodrich Radial TAs on rear. So 26.60 inches tall. The key to this pass was that she was turning around 4500 rpm in second gear (1.52) when she hit the 1/4 mile marker and shifted to third precisely at that moment. That calculates to exactly 2.56 on one calculator. So then i ran a different calculator that gives me rpm for this combo and out came 4,492.81 rpm.

The car has gone from 27.40 inch tall to 26.60 inch tall to the current 26.10 inch tall rear tires. So I am now 1 inch shorter than factory stock (27.10) I am currently coming up with a effective gear ratio of 2.66

 

Just remove the rear cover and the outer diameter of the ring gear will have the part number and tooth count of each gear and even date of the gear set stamped into the metal ring.

IT is a whole lot easier than guessing. PLUS if you have not changed the oil in a while you can do that also.
Jim
JD

 

Thank you Monzaz, much appreciated.

When i first got the car I asked my mechanic to check rear end fluid. But my concern was more quantity than quality. He checked via shortcut never removing cover. Adding this to my current to do list, its getting longer... Hoping to get some passes in before season over.

 

Why not just rotate the driveshaft 10 times and count wheel rotations? Beats having to open it up or guess from trap RPM, too much TC slip to rely on.

 

Because it's a good idea to change the fluid and reseal the cover from time to time.

 

GPS tested speed vs tachometer readings recently. 2000 rpm with the current 26.10 inch rear tire height and its TH350 was 60 mph on GPS. Confirmed at several different moments. So that backs up my first test and it looks like my car does indeed have a 2.56 rear.

I hear you Johnny, and will do ...

Yes, I have the new gasket just need to pick up the differential oil.

The final proof check should be the charm.