The Topic: You want a question?

That would be the 'speed rating.' In other words, the top speed the tire is rated for.

'Z' rated tires are rated for over 149 mph, and are generally meant for sports cars.

Interestingly, when Z-speed rated tires were first introduced, they were thought to reflect the highest speed rating that would ever be required. Since that time the automotive industry has found it necessary to add W- and Y-speed ratings (indicated in the tire’s service description) to identify the tires that meet the needs of new vehicles that have extremely high, top speed capabilities.

For the curious, W is rated 168mph, Y for 186mph. If these numbers seem odd, convert to metric and remember that speed limits are not as tightly enforced in Europe.

For my own special ultra high-speed super pursuit needs, a custom tire was of course developed. The engineers originally wanted to give the most recent iteration the rating '327-ZZZ', but Bonnie thought that sounded too much like a stripper's bra size.