I have an 8 1/4 rearend under my 68 barracuda. I did the old "jack it up and turn the tire" to get my gear ratio.
This is how I did it:
Jacked up both rear tires. (I have suregrip)
Chalked my tire and the floor.
Made 2 full turns of my tire.
Counted the 1/4 rotations of my drive shaft by...