Increasing Gas Mileage

[HUMMERcustoms.com/TAZ]

I made the following upgrades and am very happy, until I brake down and put on a supercharger.
Cold air intake
Throttle bodyspacer
Proflash computer programer
Flowmaster true duel exhaust. I used 2-40 series flowmasters and ran 3" from cats back to muffler and to axle and then had to go down to very short piece's of 2 1/2 to get over axle as it was to tight to get 3" all the way out. It made the flowmaster tech happy as he keep harping we would be taking the breath away from engine, which i understand what he is saying but it is running good. Besides Magnoflow is less restrictive than Flowmaster and they do not worry about breathing as their muffler is almost a straight thru muffler. I hope this helps but there are many more experienced guys than me as I have had My H2 since May 28, 2004. Many of the things I did I knew to do from other trucks I upgraded but, i wanted the vetrans feedback as this is my first big treat to myself in my whole life time. Listen to what these guys say. I'm lucky I stumbled onto this site.
Congratulations
Rick
Ps I forgot to tell You I'm seeng about 1.5 MPG increase as long as I do not let My lead foot take over. It is hard now not to hit throttle as it runs so much better and sounds better than stock it amases me. I'm trying to get time to dyno it. The guy at the performance center who designs and builds himself said if you just calculate manufacture claim in to what I added I'm somewhere up to 385 to 400 on HP. He leans to the 400 Hp after taking it out and driving it. Guessing for now but again that is just adding manuf. claim and we cut that a little or round down you may say. His specialty is Corvette but started working with H2 when 03 models came out with people complaining about power.