Follow the link below to help calculate the proper tooth count for your application
Time Required: 30 minutes (approximately)
Mustang 1979-1993 GT
Mustang 1979-1993 LX
Once you change the gears in your differential, you need a new speedometer gear to recalibrate your speedometer. They're inexpensive, and only take a few minutes to change. These Recalibration Gears fit 1983-1995 Mustangs.