The 99 is you load rating and W is your speed rating, which is good up to 168 mph. The tire width depends on your rim size. Look on the sticker on the inside if your door jam and it should say what the rims diameter and width are.
Once you know THD width the you can figure out the max tire width the rim will support. Discount tire and NTB both have good tire and wheel information on their websites.
chevy runs deep, Don't step in chevy!
67 Coupe 289 - TCI C4 w/2k stall; Holley 4150 570cfm; Performer intake; mild cam; Pertronix HEI; 302 heads; Roller Tips; Hedman Tri-Y headers; H-pipe w/Flowmaster 40's; 9 in posi 3.50; 205/50-17F; 225/50-17R.