I know what SS is, I know what the 1500 on the side of the trucks mean, but what does the Z mean, and how do they figure out the numbers?

Corvette - Z06
Cavalier - Z24
Lumina/Monte Carlo - Z34
Camaro - Z28
Silverado - Z71

I thought it was related to car generation, but the Sixth Gen Vette hasn't arrived yet, the cavalier only has 3 Gens, and the Lumina was replaced by the Malibu/Impala.

It all started in 1967 with the Camaro, Z28 was just a option code, and it stuck.
Same thing with ZO6 it was a 60's option code that they turned into a model name.
The other "Z's" just spawned off of them. Z34 because a Lumina was larger then a Camaro and Z24 because a Caviler was smaller then a Camaro. :-)