Not quite sure of gas mileage effect, but I do know, you probably won't pass emissions because of the muffler. The muffler should actually give you "more" power, because it is less restrictive then the stock or dual chamber styles. Think of a motor as this: It's a big air pump. The more efficiently you can get air in, and out, the more power you will make. So, you will help the air (exhaust) get out easier.