How does one decipher the FoMoCo P/S belt number in order to know its length? I am using a NAPA 25-9480 belt on my 1967 GT 390 and I know the length is 48" The catalogs indicate I should be using a C5AE-8620-L but it is way to short. Obviously it is not 62" Maybe a better question is what FoMoCo P/S belt is 48" long.
There are three different PS belts for 67 390, the correct one depends on your combination of thermactor & a/c. If you know the application, the MPC lists the size. For the first one listed in my example images, you have a ps belt, service part C9PZ-BY. The size is listed as 1/2 x 44-5/8. There is also a code, JC-445. In the second pic you will see how to decode this number. It's simple - J=v-belt, C=1/2 wide, 445=44-5/8 circumference.
If you have a pn, you can look up the service part number. In the third pic, C2AE-AA is serviced by C9PZ-AE. You then scan through the first portion of the chart to find the J code equivalent for C9PZ-AE, which is JB-456. This translates to 15/32 x 45-3/4.
This is how the MPC is set up. Oc course, it was not intended for historical representation of what rolled off the assembly line. No telling if Ford made "close enough" substitutions on service part lengths. After all, is 1/8 inch really going to matter when you've got an adjustable bracket?
BTW, the NAPA belt you listed is a 48 as you stated. None of the three belts listed in the '75 MPC for a 67 Mustang 390 are 48 inches.