Skip, I changed over to the Borla when I had 400 miles on my 04 X. So I really cant tell you if there is an improvement in the milage. I also installed an AirRaid cold air intake. The absolute best milage I can get is 20 mpg. Thats not going over 55, windows rolled up,perfect conditions. I doubt you will see any mpg improvment because you will be on the gas more after you get your Borla installed Good luck with the Borla. You will like the mellow sound it produces.