My mustang is a 67 coupe, it's not a GT. I'm wondering if it has a regular dif, limited slip, or posi-trac. I'm changing the gear oil in the rearend an am trying to see if I need an aditive for limited slip and posi rearends. I also live in Ohio and am wondering if i should use 80w-90 weight or 85w-140 gear oil. Any help would be much appreciated.

 

If the tag is still there look for an "L" between the ratio. 4L11 means 4:11 with trac lock

 

Jack up the rear end (both wheels off the ground). Turn one wheel. If the other turns in the same direction, then you have some sort of locking mechanism, like Trak lok. If it turns in the opposite direction, you have an open/regular diff.

 

Addl instruction... Car must be in neutral for this to work.......

If the driveline wont free wheel , the wheels will turn opposite each other anyway.

This check is not ALWAYS foolproof but it usually works.

 

Thanks, from what I can tell it's a regular dif. About the oil. Should I use 80w-90 or 85w-140 weight gear oil?

 

I don't think you'll need anything higher than 75W-90, if you go 140 it is much thicker and will be much thicker until heated from driving, imagine cold temps in OH and it would probably be like glue most of the year. I think 75W90 was stock, I'll check my manual.
From an oil manuf:
75W-90 gear lubricants are defined as multi-viscosity. This means the gear lubricant
has enough viscosity to protect against wear at high temperatures, as well as good flow properties at cold temperatures.
Jon