I live in San Diego, also. I noticed a drop in my MPG about six weeks ago, as well, to the level you reported. The early morning temperatures, even in our region, are significantly colder than during most of the year, and we've had substantially more rain this year (following 180 days of NO rain). I notice about a five mile per gallon drop in mileage on wet roads, and several additional miles per gallon drop based on temperature. It may be that you are noticing similar variations for these reasons. A few weeks ago, to combat winter, I did the cardboard radiator block that you'll find posted, and it has consistently boosted my mileage by 10+ MPG. In fact, my SD-LA commutes, which often netted me no better than high-50s MPG are now consistently giving 73 MPG, and last week I got 86 MPG southbound with a draft assist during the last 25 miles.