decoding rear gear numbers

[dab67]

OK CRG once again I need help.
 In a previous thread I ask for help in trying to determine my rearend gearing because my speedo was off by 10mph on the high side 50 was actually 60, 30 is actually 40.  It is suppose to be based on the numbers BP0403G2 a 2:73 or 3:08 built April 3rd in Detroit on the 2nd shift. I just had the car on a hoist and attempted to do it the old fashion way, mark my rear tire, put it in neutral ,marked the drive shaft and turned the tire one revolution and counted the turns on the drive shaft. Three times I did it and the shaft turned only 1 and 1/2 times!!!! We pulled the rear cover off the rear housing and I got these numbers off the ring gear, not sure this helps any, 1GM 3910823  underneath this is 15 11 ( or 41 ) 2 67. Is the 3910823 just the part number for the gear or what? Based on the old fashion turning I have a 1:1:50 rear gear!!!!!!!!!! Is this possible??
Help !!!!!!!!!!!!!!!!!
dab67

Dave

[bertfam]

Dave,

Based on the code you supplied (BP0403G2) and the numbers stamped on the gears (15-41), you have a 2.73 rear axle ratio, but something doesn't add up here. Your ring and pinion are dated February, 1967 but the BP axle code in 1967 was a 3.08. Either you have a 68 or 69 rear end, or the gears have been changed out. Check the date code on the "pumpkin" to see if it's 1967, 1968 or 1969.

Ed

[dab67]

Ed:

I have been told not sure how true it is, that the BP code may have been used in both 67-68.the speedo gear for what it is worth is the same for the 2:73 and 3:08, 21 teeth.  But the only place I find the code listed is under 68. The dates on the ring gear run in line with the build date of my car which was the first week of April 67. But what I can not figure out is why the drive shaft only turns 1 1/2 times for one revolution of the wheel. It makes no sense!!!!! What am I missing??

Dave

[JohnZ]

You have to turn BOTH rear wheels one full revolution when you count the driveshaft revolutions to determine the axle ratio, so the ring gear makes one full turn. 

[dab67]

OK John:  Now I am really going to act stupid (WHAT'S NEW!!!) Why is that you have to turn them both? Why doesn't the ring gear make a full revolution or turn when only turning one wheel? Would ii be a good guess based on the 1 1/2 turn of the driveshaft with one wheel that the gearing is 3:08?

dab67
Dave

[KurtS]

A differential is a gear driven adding device. 1 output rev of both tires = (ratio) rev of input.  Only 1 side turning, only 1/2 of (ratio) input rev. 2 revs of 1 wheel = (ratio) rev of input.
Pretty need when you get into the gear theory behind it.

The ratio does show up here.
http://www.camaros.org/drivetrain.shtml#AxleCodes

[bertfam]

Yes, the 67 BP is 3.08, but his 15-41 stamping indicates a 2.73. That's what doesn't add up.

Ed

[JohnZ]

Gear swap - the numbers on the ring gear confirm it's a 2.73.

[dab67]

Bert and John: 
What if the number is 15-11 and not 41? Not sure if it is 11 or 41. Would that make a difference? Also, from what I find, the speedo gear in the tranny for a powerglide is the same for a 2.73 and a 3.08 correct?
Thanks for the great info.

Dave

[Jerry@CHP]

Dave,

It can never be 15-11.  These numbers refer to the number of teeth on the gear set.  15 teeth on the pinion and 41 teeth on the ring gear, divide 15 into 41 and that gives you the gear ratio.  Try this one, 7-41, that is what I have in the Old Reliable race car.

Easy enough?

Jerry

[sd1968z28]

5.86 gear that low!  what kind of rpm does it turn through the traps.  my 71 camaro has 5.43 with a 33 inch tall tire it runs about 7800, your must be screaming?

[Jerry@CHP]

8100 in hot weather and about 8400 with good air.  Stock rocker arms I might add.

JM

[sam]

Stock rockers.........and intake and carb and stock cam shaft lift, stock piston weight etc,etc! That car flat out runs despite NHRA's tuff weight break for the 302 chevy. And besides JM is not a bad driver either.

[dab67]

Bert and John:

What would the reason be for someone to swap the gear out? the 3:08 is already a crusin gear why the 2:73? Could it have been an error at time of production? Rechecked and it definetly is 15-41.

Thanks for all your help

Dave

[Jerry@CHP]

I know of many errors in the rear end ratio dept.........first hand.  The survivor 67 Z28 that I own was ordered with 4.88s but came through with 3.73.  A supply issue according to the original owner.  My best friend ordered a brand new SS396 Camaro with 3.73 and for some strange reason car was built with a 3.55.  He was never told anything by the dealer or assm plant.  Two examples and I have many more.

Jerry