For this achievement you will need to drive 20 different vehicles throughout the course of the game. Jump into every vehicle that you come across to have it sent to your garage, or "vehicle lot", so that you may use it in the future and also make headway on this achievement.
Regular Vehicles:
Forklift, Street Cleaner, Steamroller, Backhoe Loader, Sports Bike, Motorcycle, SUV, Muscle Car, Sedan, Sports Car, Police Car, Hearse, Low Rider, Taxi, Sedan [Student Driver], Van, Fireworks Van, Ambulance, Party Van.
Combo Vehicles:
Mobile Locker, Junk Car, Turret Rig, Mini Bike, Forkwork, Rollerhawg, Junk Truck, Party Slapper, Junk Bike, Shockdozer.