Possible Clues:

• Vehicle going fast with Shell
• Means to travel quickly? It's hard
• Vehicle quickly becomes a hardtop
• Shell of a tortoise
• It's hard to get a vehicle over a certain speed
• Shell hitting vehicle going quickly
• It's hard for a tortoise to have speed of 23 down
• Shell is what gives a car a speed that's hard outside
• Beetle speeding, though bit of a 14
• Shell on back of tortoise or crustacean