Completing challenges with any class will gradually earn you leaderboard ranks and persistent upgrades linked to your player account, making you the star of the show and a badass in your own right. (dealspwn.com)
As these quests are completed, you'll earn currency to spend back at the armory for persistent upgrades. (snackbar-games.com)
io game, Woopdrift actually allows you to use the money you collect to buy persistent upgrades such as improved vehicles and different color schemes for your car. (crazygames.com)