I just refilled at Costco, Tustin, CA on November 2 and immediately saw mpg drops ever since (for 2 days). In my communting route, I can easily achieve over 60 mpg. But in the past 2 days, I have to try hard to maintain 55 mpg. I don't fill any less power. It has to be the gas. Does CA have winter blend gas starting in November every year? Or else.