I have an 05 R* Silverado too and was wondering if anyone has done a stage 1 exhaust-only modification to stock mufflers and got good results for MPG. How is your low-end torque as compared to before the stage 1 was done? Did you only do the exhaust mod without rejetting? I was thinking of doing an exhaust-only mod without rejetting too, but some say the engine will run too lean and lose low-end torque without the backpressure. I did remove the AIS (Canadian model with minimal tubing) last night and really love the improved exhaust tone and not hearing that crappy chirping sound from the AIS at idle and low speed touring.
The mileage increase you ae experiencing only makes sense to me, if the engine has to work less hard to fight backpressure then the more mileage you'll get with the added efficiency. Anyone else got any thoughts on this?