I believe, and the chart at Bridgestone verifies this, the 285 tires do not have a speed rating. Speed ratings symbols are not required on light truck tires. Therefore, they probably fall under the R rating which is 206 mph (used for HD truck tires). This would seem logical, since GM caps the top end of all their vehicles about five miles under the tire rating (unless you have a ZO6
)
H rated tires are usually capped at around 125.
In addition a tire is good shape will not blow up if pushed past the speed rating, and a tire that is not in good shape (especially under-inflated) may blow lower than the speed rating.