Unlockable Vehicles (also Key Cars) are the vehicles, which can be unlocked primarily for Free Roam, but also for the certain episodes (missions).

Season 1 Key Cars

Episode 1 (Fast Cars): Vengeance
Episode 2 (Special Withdrawal): Funk Monkey
Episode 3 (Bay City Heatwave): Shadow (black)
Episode 4 (Middle Man): Stallion
Episode 5 (Squealing Piggy): Vengeance GT
Episode 6 (A Little Mayhem): Shadow (brown)

Season 2 Key Cars

Episode 1 (Not So Easy Rider): Moonshine
Episode 2 (Swann Dive): Skunk
Episode 3 (The Departure Gates): Daisy
Episode 4 (Wheels with Wheels): Katstander (black with flames)
Episode 5 (The Big Score Part I): Bullet
Episode 6 (The Big Score Part II): Dawn Cruiser

Season 3 Key Cars

Episode 1 (Breakout): Katstander (yellow)
Episode 2 (A Parking Ticket): Neptune
Episode 3 (A Red Tomato): Funk Monkey GT
Episode 4 (Save the Senator): Shadow (blue)
Episode 5 (A Bomb Surprise): President
Episode 6 (18 Wheels of Trouble): Lady Blue