It will only drop your city mileage if you are hotrodding. Backing off star wheel so its all the way towards the front of the motor will allow fueling much sooner which will make boost sooner and it will smoke more. You do not need to slide anything to take advantage of the extra performance gained by this adjustment. my advice is take it all the way forward and then move it the other way a little at a time to get to the place where performance VS smoke compromise is something that suits you. Sliding the AFC housing forward will help also but is not required