As the number of tire offerings for Corvettes at Mid America Motorworks increases, I have been asked by many of our Mid America Motorworks customers, just what do the markings on our Corvette tires mean?

http://www.mamotorworks.com/corvette-1-297.html

I found most of what is needed in one of our trade journals published by National Business Media.  I will repeat and customize that article for our Corvette tire customers.

Speed ratings are based on laboratory tests where the tire is loaded against a large diameter metal drum to reflect its appropriate load, and run at increasing speeds in 6.2 MPH steps in 10-minute increments until the tire's required speed has been met.

When Z-rated tires were first introduced, they were thought at the time to reflect the highest tire speed rating that would ever be required, with the Z rating indicating that the tire was capable of speeds "in excess of 149 mph."

285/35ZR19 Mid America Motorworks Corvette Tire #638-829

285 = nominal width of the tire is approximately 285 mm at the widest point

  35 = the height of the side-wall of the tire is 35% of the width

    Z = speed rating 149 mph+

    R = radial tire construction

  19 = the tire fits a 19 inch diameter wheel

Eventually, the tire industry added W and Y speed ratings.  A Z rating may still appear on the tire on its own (indicating a rating of 149 mph+), but may also appear in addition to a W or Y rating symbol (with W indicating a rating of 168 mph; and Y denoting 186 mph).  To make matters even more confusing, some tires may feature a rating symbol of Y (following the load index), with the load index and speed rating encased in parenthesis.

Example: 285/35ZR19 (99Y).  If the Y is seen within parenthesis, this indicates a speed rating "in Excess of 186 mph"

Other speed ratings:

 R  106 mph
 S  112 mph
 T  118 mph
 U  124 mph
 H  130 mph
 V  149 mph
 W 168 mph
 Y  186 mph
(Y) 186 mph+ 
 Z  149 mph+

Tires referred to as "ultra-high performance" include those featuring V, W, Y, (Y) and Z speed ratings.

I hope this helps somewhat for those of you who inquired about speed ratings!