My car usually fills up with about 16 gallons of 91. I get about 300 miles per tank and I usually drive aggressively. 30percent street 70 highway.

 

You drive a G. Spend the extra few cents for 91. In the long run its a lot better. I always fill up at shell 91

 

Fill up with the best gas all the time, I get about 280-310 to a full tank 17-18 gallons. I also had a bad o2 sensor, the post-cat on the drivers side, but replacing made practically no change. I drive 15/85 city/highway, so the mileage is not too bad, but 11 to the gallon seems pretty low. Check for holes in your tank