Probably the number 1 reason for a 2" tail pipe is---cost. 2" pipe is cheaper than 2.25 or 2.5". You can get away with a smaller tailpipe because the exhaust gases are cooler and have contracted somewhat compaired to right after the manifold. This is especilly true in a non-cat car or truck. Look at new trucks and suv's the tailpipes are much bigger then in the old days. I believe that volicity is the key to a good system. A too big ex pipe will let the gases slow down and hurt performance. This is most noticible at low speeds on smaller engines. This is why IMHO people think that they need "backpressure" to make it work. When the smaller system was put on volicity went up at low engine speeds and helped scavage out the ex gases. After all why doesn't a high end car like a new Corvette have a 5" exhaust system? Jay 6155
