Nothing says "I'm a poser douchebag" faster than a Ferrari or BMW tie/t-shirt/hat. This is doubly true if you don't own a BMW or Ferrari. Real Ferrari owners don't drive around wearing $12.00 polyester shirts with big ass stallions on the front, they wear $5,000 pants. C'mon! You don't need to advertise when you're driving around in a Ferrari, the car kind of speaks for itself. You only need to advertise when you're driving around in a Tercel.
The one minor exception to this rule, and we hope Tocsin would agree, is a tee proudly proclaiming allegiance to a novelty car that you currently own. If you drive a Trabant, and someone tracks down a Trabant mechanic jumpsuit you're more than welcome to wear it.