Need for Speed: Carbon Message Board  

Thats because you have no tier 2 parts. Here's a chart to show you how the part system works:

Tier 1 parts - Tier 1 Cars
Tier 2 parts - Tier 2 Cars
Tier 3 parts - Tier 3 Cars

Of course you know that the order of preformance from worst to best is Tier 1, Tier 2 then Tier 3 but you unlock the cars in pretty much the same way (except when you start in Angie's 21st Street). If your still confused PM me and tell me what you don't get.