In San Antonio, Shell seems to be the best. Valero (which is virtually everywhere, since the HQ is here), varies from tankful to tankful -- sometimes it knocks, sometimes it doesn't. I also use Exxon from time to time. It's about halfway between Shell and Valero.

Actually, the best gas mileage I've ever had has been on the Kansas Turnpike. I was able to get 32 MPG out of my '94 Thunderbird there (it usually got 28-30 on the highway) and 29 out of my '00 Mustang (usually 25-27). The surface of that road is incredibly smooth -- as it should be, since it's fairly costly to drive it. If anything, I'd say road surface is the single biggest difference (excluding wind, of course) there is when it comes to mileage.