given the amount of regulatory gates you must pass through to purchase a car (license, registration etc), I doubt that criminals are going to use Bitcoin payments for criminal services to purchase a car.
They are far more likely to convert to fiat currency and then purchase the car.
There are companies, such as Chainalysis that specialize in analyzing the graph created by mixers/tumblers. If you want a coin that truly cannot have it's origins traced, use Monero, since it's a completely different structure with a public/private "view" key. It's actually a very interesting system.