Tire/Spedometer?

There are a number of different ratios found in the 7-1/4" differential; I know that this was not what you were hoping to hear. Check out this link and you will see the range from 2.26:1 up to 4.11:1. I do not know how many were used in your car.

http://www.differentials.com/pdfs/4.pdf

You may simply have to jack up the vehicle and manually spin the drive shaft to see how many times around it goes in order to get one wheel revolution.