I have been running the Borla true duals: headers, into two mufflers, with two side-exit pipes, for about 4 years now (97 2500 4x4 auto w/LWB). Didn't notice much of a difference, to tell you the truth. No change in mileage. Now the Paxton supercharger I now have....that's a different story. Also, jsut had two new high-flow cats installed - again, not too much of a difference.