Reference: C15 s/n 2842129988 history and restoration

[Tony Smith]

Originally posted by Alex Blair
The 637 in the number refers to the gear ratio..the C15's had the 6.37:1 rear ends and the C15A had the 6.39:1 rear ends..diffs,if you prefer..

The first C8A,C8AX also had the 6.37:1 diffs... but changed down to 7.38:1...on replacement...

Slight correction to those numbers, Alex, they refer to the ratio, but expressed as Teeth to teeth, not as a numeric ratio.

6:37 = 6 gear teeth on the pinion and 37 teeth on the crown wheel, or a numeric ratio of 6.16 to 1
6:39 = 6 tooth pinion on a 39 tooth crown wheel = 6.5 to 1
6:43 = 6 '' '' 43 '' '' '' = 7.16 to 1
7:38 = 5.43 to 1.