The reason for poor mileage is mostly the use of improper engine oil, which restricts the vehicle while driving and makes more strain on the engine during the rides. The use of dirty air filter restricts the air flow into the system, making the fuel burn with less than required air, leading to residues of fuel from the car. The injectors when not cleaned properly can affect the pickup of the car as fuel flow also gets restricted with it. No doubt, air conditioner puts a lot of stress to the car and that cannot be changed with these measures, but overall driving feel and mileage varies greatly with all the above changes. You can get your air filter cleaned in the middle of your service interval and the injectors are cleaned periodically the service centers also. Always use a high quality engine oil and get the petrol filled from a renowned and trusted petrol pump only.