Meh... This is like when folks change tires and their gas mileage goes up. Usually it is one of two things...
- Their driving habits changed because they are now aware of the driving compared to before... Just drive...
- if only the muffler was changed, yeah it is possible for the mileage to change in either direction or not at all. Some give less back pressure allowing for the engine to RPM up faster (Allowing for less throttle and better MPG) while some have this same effect but in a negative manner (Computer thinks that it needs to dump more fuel in because of the RPM there by wasting some fuel).
So to answer your question. It depends on all the other add-ons (Filter, computer etc) not just the muffler. And most importantly in our RT and RS cases... If the engine has been broken in yet. More times than not folks see a better mileage after a few thousand miles on their rig. Good chance this may be when some folks have also changed mufflers.
On the other hand... The new muffler just might work better for them in their road and environmental conditions they ryde in allowing the engine to perform like it needs to... :dontknow::thumbup: