KLrx7

I am wondering what kind of gas mileage I should be getting. I am only getting about 240 miles to a tank, about 15-18 mpg. When I bought it the idle was rough and engine light came on after first week. It also failed emissions. Took it back to where I got it and they replaced O2 sensor and cleared codes. Idle is better but engine light came back on a week later. I found an emissions receipt from the previous owner and it barely passed and was giving a code for intake air temp sensor. I just replaced the sensor and still no difference. Any ideas would be appreciated. Thanx