Regarding fuel average my vitz driven 55000 km is giving me around 12 city driving which is good enough considering traffic jams / routes etc. clean air filter makes hell of difference in mileage. The silencer sound common problem same as u explained come from the spring bolts of the rear silencer , get the bolts oiled from the silencer work shop problem will be solved. Sound from brake is from the rear pads , I have got them changed , then cleaned them once the sound disappears but comes back again after a couple of days. Fail to understand what else to do so have decided to live with it. Knocking in city driving is controlled by driving the car in S mode , a little compromise in mileage but u r done with the knocking.