I have an '81 Mark VI and I am planning on getting posi. Can anyone help me to find out the measurements of my rear end? :nos:

I have an '81 Mark VI and I am planning on getting posi. Can anyone help me to find out the measurements of my rear end? :nos:

It should be a Ford 8.5 Inch Rear I believe. or are you asking about the gear ratio?

http://mustangmonthly.com/howto/p98387_image_small.jpgSuppose you bought a vintage Mustang and you’re not sure what gear ratio is installed in the rearend. You can simply plug a few numbers into this next formula to determine what gear ratio you have now or to determine what gear ratio you should be running for the rpm range you want to be in at a certain mph. This is the formula for determining gear ratios.

Are you sure about the 8.5 inch for te '81?