I just purchased a slightly used 2003 dealer-driven Suburu Outback Legacy which has 5000 miles on it. It supposedly should get around 20mpg city but I get only 13.5 mpg and I use the premium gas I'm supposed to use. I'm a good driver and the car doesn't sit idling or anything like that. Why would I only be getting 13.5mpg? I've checked the mileage on two different fill-ups so I'm sure this is the mileage I'm getting. Should I bring it in for service?