it will get a bit better as the motor breaks in. there is generally a 3000 mile break in on a motor. i know with my tacoma which i bought new it got a lot more powerful feeling around the 5000 mile mark, and the gas mileage was getting better all the time until abut that 5k mark. keep in mind that 15 is the average, which is gonna be terrible until you get on the freeway even a little. i know with the lexus i had, when i filled up and started drving it would say like 12 until i cruised for a few then itd go way up.