2.5" would be more ideal for a TT. 3" is actually overkill for a TT unless you're pushing BIG horsepower. Read the little tidbit on specialtyz.com. I could've went with 2.5" on my car, but I don't plan on pushing a buttload of horsepower. My car's already one of the fastest cars in my area, plenty fast enough to get me in trouble. And no, I'm not adding hardpipes for horsepower. I might gain 1 or 2 hp due to smoothed out airflow, but nothing significant.