It depends on what wheels your cougar came with. If you have the 15 inch, six spoke fan blade wheels, you need 205/60 tires, if you have the 16 inch, sport package wheels, you need the harder to find, 215/50 tires. 17 inch wheels were not available in '99 to the best of my knowledge. If you have trouble finding the 215/50/16 tires, both the more common 225/50 tires, and the 215/55 tires can be used without much problem.
215/50r16
Pretty sure its 34mm
215 75 r15 The correct tire size is listed in your owner's manual and on the driver's door post.
For the 1999 cougar they have 2 different size brake rotors it sounds like you need the 276mm size
my tire sizes for my cougar are P215/50R16 or go here so you can find your size http://www.dragtimes.com/Mercury-Cougar-Tires-m31-n237.html
5X7 all around
What is the motor size? Run1
205/75R14
What is the motor size? Run1
6 x 8
5x7 or 6x8, either or
4x6- not upgradable without sheetmetal modifications.