This question comes up a lot. Instead of it being posted over and over, I figured I would answer it here.
The Trailblazer suspension has an overhanging upper ball joint.
While it provides an excellent turning radius for the vehicle, it inhibits the user from going too large with the tires without making other modifications.
From what we at ORTB have measured and other people have reported, the maximum tire diameter without modification is 30.5".
This means one of the following tire sizes should fit without any modifications:
17"
245/70/17 = 30.50" diameter
255/65/17 = 30.05" diameter
265/65/17 = 30.56" diameter
285/60/17 = 30.46" diameter
16"
245/75/16 = 30.46" diameter
255/70/16 = 30.05" diameter
265/65/16 = 29.56" diameter
285/65/16 = 30.58" diameter
If you go over 30.5" diameter, we suggest wheel spacers and a lift. See the other sticky in this section for more information on wheel spacers.