I have mine locked open using the JB4, prior to that I unplugged it in the trunk. My butt dyno can't feel any effect one way or another & I don't see any effect on mileage. I just like the sound.
2013 BMW 135is 6spd conv - 2011 BMW 335is Coupe JB4/ISO
2010 BMW X3 - 2009 Ford Expedition 4X4 EL - 2009 V-Star
2003 Nissan Xterra 4X4 - 1998 Ford Ranger 4X4 Splash