i came across this while looking for stainless steel tubing -


     FACT! Before purchasing an oversize 2-1/2" exhaust system give it a little thought! The problem with 2-1/2" exhaust tubing (a common size mistake) is that the velocity in the system decreases with larger diameter tubing. This reduction of velocity is caused by the exhaust expanding in the larger tubing. This actually produces more back-pressure by slowing the flow. The result of this is a significant decrease in low end (2000-4500 rpm) performance of your Toyota's engine.


wow, i guess toyotas use different laws of physics :-)  i thought the backpressure squared was proportional to the airflow velocity.