I installed V&H Monster Round Slip -ons to my 2008 Road King Classic. The bike is all stock. I absolutely love the sound but my gas mileage has dropped about 6 MPG. Is this normal? I was told the ECM would adust to a minor mod such as slip-ons but not to more air. Overall the bike seems to have more get up and go and runs great. I will give up the gas mileage for power and sound but I am just curious. Thank you!