I haven't noticed any real difference in MPG, the pipes I put on (Speedstar Dragster longs) and they have the O2 sensor mounts. I highly recommend keeping the O2 sensors intact as this is a large part of the CPU adjusting the mixture correctly. Even after the BAK installation the CPU adjusted itself to the new setup in a matter of seconds. Just start it up. Let it run for about 30 seconds. Turn it off and when you restart it is all adjusted and ready to ride. Don't forget to disable the AIS unless you like the kaboom sound out the tail pipes. I didn't remove the AIS due to warranty problems but I disabled it by cutting the AIS hoses behind the rectifier. Plugging them with plugs bought from Lowe's, and taping them back together, to look stock to casual inspection. I love the way the bike runs. Seems to have alot more torque and the cagers don't ride in your right side blind spot!