The Roadster is a orange Gavin Roadster X3 in Dead Rising 2. It can be found on display in the Royal Flush Plaza, and can be driven into the Slot Ranch Casino and food court - and if the blue maintenance doors are open, all the way to the vent that leads to the safe room.
This car is not recommended to get Achievements like Z-Genocider 2 or Z-Genocider Master: it is fast, but not particularly durable. The SUV and Slicecycle will kill large numbers of zombies far more efficiently.
However, this car is extremely useful for the Hero of Fortune Square achievement, since it can rapidly support survivors, especially injured ones. Even if Chuck is escorting multiple people, it will transport one, and clear a path for the others.
The Driving Magazine will triple the durability of this vehicle.