Bro the RPM screw that you are referring to is just an idle setting screw. in theory it shouldn't change your mileage because its only function is to ensure that your bike maintains a certain level of RPM at idle when you are not twisting the throttle. as far as air-fuel mixture screw is concerned, it is also at some discreet place on your carburetor. this is the screw which decides how much fuel your bike will burn and how much power it will produce. the trick is to first quantify/analyze your driving needs then set it accordingly.