California has switched from MTBE to 10% ethanol early this year. I doubt you will see any mpg change. It is quite likely you have been using ethanol and just not noticed.

I've got 2 fillups on my TCH and am averaging 42mpg. I've got 519 miles on the 3rd tank and the tank average is showing 42.9mpg. I drive about 80% freeway, mornings have light traffic with occasional slowing, evenings are rush hour traffic, often taking a hour or more to go 25 miles.