It's actually well know among smog techs I've worked with that Oregon and Nevada gasoline nets higher mpg. The measurements are probably skewed somewhat by the fact that the only people checking are returning from freeway trips and have a lot of nice long downgrades on the trip home, but there's also a noticeable difference in the fuel trim readings with a scan tool. Out-of-state gas requires shorter opening times from the fuel injectors.

Basically, California has a pretty screwed up blend and it's required to be that way by people who don't actually understand the way cars work.

But Teri's gas mileage is exaggerated by the highway end of it. We're still waiting to see the fuel economy increase with the trip up compared to the trip back. Unfortunately, she still had a lot of fuel when she got back, so that's been all in town driving.

To give you an idea of how screwed up California smog stuff is, the test for my smog license actually has questions on it that will cause you to fail the test if you answer all of them correctly. They teach us the "correct" answers in our smog classes so we know to look out for them.