You can go to the American Muscle web site. Then find the gears for the 2010 Mustang
v6, see what rpm's you should be doing at what miles per hour with the gears you choose. Doing this should give you a good ideal on what gears you have. This is what i would do to find out what gears i had if i didn't know.......
Little Red. 2011 V6. Automatic Trans., C&L CAI, JLT Catch Can, BBK Ceramic Coated Long Tube Headers with BBK Catted X Pipe, Axle Exchange (American Muscle) Aluminum DriveShaft, GT 500 Lower Control Arms, Koni (orange) Dampers, Steeda Watts Link, Ford Racing 3.73 Gears, Magnaflow Street Series Axle Backs and a few more mods....Wifes car, 2014 Ford Edge Limited.