I am a newbie to the truck club, and a newbie to my Suburban. I have a 1996 GMC K2500 4X4 diesel 6.5 L. Presently I am getting around 14 MPG in the city, and 20 MPG on the highway. I would like to upgrade the exhaust system. I purchased the vehicle used so I am not sure if the system I presently have is in fact already upgraded. The present system is a 2.5 inch crossover pipe, then a 3 inch down pipe that continues to the muffler. There is a 2.5 inch pipe from the output of the muffler to the rear of the vehicle. Is this system the stock system? If this is the stock system, what would be a good system to upgrade to, meaning what size pipes from where to where, including a better flow muffler? I do not want to go exotic, but I do want a better exhaust system. I know I should also upgrade the air intake system. Not sure if asking for recommendations for the air system should be in this thread, or generate another thread. Any thoughts on that?