11km/l on ac is quite reasonable considering city traffic, however on highways it seems a bit low.
Low mileage could be related to ignition system, including spark plugs, wires, etc. get the wires tested for resistence, if one of the wires have any burn marks or cracks this would happen, Get the fan belts, alternator belts checked if anything is over-tightened or lose, does the engine compartment make any additional noises? Do you use recommended engine oil viscosity..
Mostly such issues are buried deep and need thorough inspections via highly skilled troubleshooting staff.. But, sadly most of the dealerships and other automotive repair shops concentrate of quantity and not the quality of the work.. If your car starts on first self and they hear the engine running with no check lights thats good enough for them.. anything above that is way too big a burden of customer service and satisfaction..