Collect the scattered golden stars throughout the map to unlock the following rewards.

Effect - Code

Double Ammunition - Capacity Collect 30 Stars
Double Car Durability - Collect 40 Stars
Free Vehicle Upgrades - Collect 50 Stars
Health Increase - Collect 10 Stars
Nitrous Increase - Collect 20 Stars

Note: Everything resets once you get to the 2006 era and collecting another 50 Stars will unlock the same attributes. (There are 100 total in the game)