Are you carrying lots of extra weight around? That makes a big difference. Also, how fast do you drive? Do you have an auto or a manual? Do you have huge chrome wheels on the car?

My LS will see 29-31mpg in the city and 34-36 on the highway, though, with fewer options, my car weighs less and will get slightly better mileage that way.

Don't bother with premium fuel if your car is designed for regular unleaded, you will see no gains at all. However, if your car is designed for premium, then you will see a loss of power, and economy with unleaded fuel because the engine computer will detect knock and reduce output to protect itself.

Hope you figure out what's going on.