I have a k&n drop in filter and flowmaster muffler on my 92 hatch and I get around 36-38 mpg, but I jus picked up a ST rear sway bar. But the bar will not fit because the muffler is too close to the body. I would rather have a rear sway bar than the muffler but do u guys think I will lose mileage wit the stock muffler compared to the flowmaster?

 

Performance mufflers have a very minimal effect on fuel economy, as at lower rpms some restriction increases exhaust velocity(think airplane wings) thus actually increasing pumping efficiency. Case in point, the vx comes with the same muffler as the rest of the hb's (though the rest of the exhaust is different).

 

I thought the si had a bigger muffler than the rest of the hb models, at least the outlet for sure