3g hemi gas mileage

Well I drove 74.8 miles, took 5.391 gallons to fill it, and that come out to 13.87 mpg...this is freaking dismal!!!! However, I did throw the 91 octane tune back on and after talking to johnparts, I also put a new o2 sensor on. I will try and get the down stream o2 hooked up, but I dont think it will help much. So now I'm leaning towards changing the center out for a 3.55 instead of the 3.91's, that should help some.