It's strange, but a vehicle just won't fall apart with you at the wheel. Unimportant bits, like wheels, the roof and the back seat, might fall off, but it will keep on going, as if held together by pure willpower. But you have a long-standing feud with your mechanic.
Normally, a vehicle suffers catastrophic damage from attacks that inflicts twice the vehicle's Toughness in a single attack. In your case it takes thrice your vehicle's Toughness to cause a crash. But if damage ever exceeded twice the vehicles Toughness, it falls apart when you step out of it.