I've removed the decibel killer, and its makes quite a bit of noise for a scooter. I wanted it to make it sound better for more visibility. I haven't checked my spark plug, but like I said, I don't seem to notice much change in gas mileage, and since I'm only on about 1010miles on it right now, its just starting to get fully broken in. Been using it for a few months now, and its great. I noticed that it pops sometimes, which I never had issues with on the stock.