Sion

a typical tire size is P225/75R15S

P is for P-metric
R is for radial (as opposed to bias ply)
the last letter (in this case "s") is the speed rating

some manufacturers us this syntax P225/75SR15, placing the speed rating before the R

note: just because you have high speed rated tires does NOT mean the you cant have a blowout while travelling at a speed lower than the rating. in fact, most blowouts are not caused by speed, but rather by something in the road (debris, a pothole, etc) damaging the tire.

blowouts due to speeding most often occur while cornering, which places the most amount of stress on the tire, particularly the sidewalls.

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