Z is an older speed rating. It indicates that the tire is capable of speeds in excess of 149 mph, but doesn't say how much over 149 mph they can go. When Z rated tires were popular, there wasn't much of a need for tires that would go over 149 mph. Newer tires typically use V, W, or Y, which have the following top speeds:
V 149 mph
W 168 mph
Y 186 mph
There are also tires that have the (Y) is parenthesis, which indicates that the tire is capable of speeds over 186 mph, but don't state how much over.