My cousin recently bought a 2008 mustang just wonder what the stock gear ratio is and what size rear end is in it, thanks.

 

Assuming that he bought it new (or that the previous owner hasn't changed anything, if he bought it used)...If it's a V6, then he has the 7.5 rear end and I believe 3.31 gears. If it's a GT then he has the 8.8 rear end and either 3.31 gears or 3.55 (Package option, isn't it?)

 

Yup, Mock01 you're correct.
Most likely is 3.31's.

 

7.5" 3.31, open diff