One of the things that might effect your mileage is the type/quality of gas you use. Always try to stay with the same brand, Mobil is good, Chevron too. I would stay away from Exxon and Amoco. The idea is if you keep changing your type of gas your car is not able to adjust to the chemicals in one type and make optimal use of them. Also keep in mind that Chevron has Techron and Texaco has Clean System 3 these also help out with keeping your fuel system, including injectors clean, hence better gas mileage. Yes you pay a little more for this gas but the benefit to your engine is worth it. Especially one that can be as temperamental as ours.
Anything special about Shell gas? That's what I tend to use...been on it for about 2 months now after switching from Chevron, which I might be going back to permanently since Shell seems to be the most expensive gas.
Exxon and Mobil are the same gas company. Exxon OWNS Mobil. I use Mobil all the time. As a matter of fact, I have been using Mobil gasoline now for over 8 years and I love them. They have excellent gasoline and I like the transponder which they have that I always use. I also get 15 to 17 MPG in the very cold wintertime with my V6 Mustang. This is normal because your engine works more in the very sub-zero colder weather. When springtime and summertime comes along, I get 19 to 21 MPG. So this is not bad for a V6 that has 200HP in it. If the V6 had less HP, it would be more economical. I don't really care about having less than 200HP and getting 26 to 28 MPG. I would rather have the HP and less MPG fuel mileage. The 17 to 19 MPG is very good for the V6. You have to learn to live with it.
I filled my 1999 stang 4 days ago, with the highest octane I could find (it has been sitting for three months). I am now at a quarter tank and only put about 130-150 miles on it. I kinda drive like a bat outta hell, I also put some Seafoam in the tank to clean the gunk out.