I have a truck with a 12 bolt . Near as I can figure with a TH350 at 55mph I am only turning around 16-1700 RPM with 28" (275-60-15) tires which makes the gears very close to 2.56
Here are the numbers I found on the axle. Can someone tell anything from these numbers. Diff. is single rib, numbers on cast are E8-81, NF3896838, GM57 and a stamped number on top of passenger side tube HA 05 22W

When truck is on jack and I turn one wheel the other turns the opposite way but it will lay 2 tire tracks if you gas it at start. IS it possibly Limited slip?

 

open your glove box, there is a sticker in there that will give you the RPO code for the rear axle..... it should be a "G" code if I am correct, exp G80... then google that code and you will know your axle ratio...... good luck

 

This truck is 45 years old and probably doesn't have the original axle anyway much less the original glove box.

 

ok, sorry you did not state that in your question....... im sure someone out there will be able to help.......

 

Go to "FullSizeChev" those guys are always messing around with stuff like this.

 

Thanks USMARINE