Not good man. First cause is poor air intake. Check your air intake filter and make sure it's not dirty. I've also seen failing MAF (Mass air flow) sensor's cause this. A bad or dying cat converter(s) is another. Dirty fuel filter is another cause. Also take it to autozone or something and have them see if there are any codes. How's the car's performance?
I drive a lot of city and if that's all I do, I get around 300-330 Miles per tank and if I do a lot of highway it's around 370-410 Miles Per tank.