Is your car fresh imported or locally driven
15 is the average on lower side, it should be at least 17 to 18.
First step to ensure is
Engine oil (use only 0w20 grade, I personally am using Aisin 0w20 in all my Japanese cars)
Use original or vic oil filter(original is difficult to find)
Also check/replace air filter (though I have not replaced it myself and my car is at 67000km)
Also check the filter of battery pack underneath the rear seats, sometimes dust deposit in that area, which affect the performance of battery pack. Clean the ducts and filter.
One important step for economical driving is driving style, drive with light foot, it will make considerable difference.
Please refer to my previous posts where I discussed fuel economy of this car for further details.