Why is my car getting bad gas mileage

i have multiple questions with little time to get tehm answered first i waS WONdering why my car is getting such crappy mileage i changed teh spark plugs and my car still sucks down gas also my stock radio does not seem to be working at all as if no power or anyhting is ggeting to it and my interor lights wont turn on at all. also there is a sound coming from my rear left tire i jsut put new drums and pads on teh back and its still making the noise