I got 27.4 miles from Knoxville, TN to Cinci, OH then it dropped to 26.8 to Detroit, MI. Must have been the wind. I thought it would get worse mileage through the hills! That was doing 70-80 mph. Without resetting it and spending a week driving in the Detroit suburbs and then driving back to Knoxville, it was still at 24.5! This is my wife's LS that gets about 16.8 in the city in Knoxville.
They stink in the city. I am averaging 18.5 in my LS with mix city/hwy. I was really shocked at the good trip mileage we got with the car loaded down with crap. I always thought people on this board were exaggerating their good mileage.
Every car I have owned for the last 5 years has gotten roughly 19 mpg. From my Ranger to my supercharged Mustang. With 13.5, I would be checking the O2 sensors like stated before. Go stretch the car's legs and go on a short trip Sunday and see if the mileage goes up and up and up!