for those of us who live in Southern California, we can't assume that we'll get the same gas mileage because our gas here is oxygenated (to meet emissions requirements) and also because of our traffic. NEVER put Arco or 76 gas in your car...they use ethanol, which is crap. stick with Chevron/Mobil/Exxon/Shell (they use not-as-crappy MTBE).
...if you were driving up to Malibu and it was 40 miles from your house, chances are that for 30 of those 40 miles, you were in stop-and-go traffic - that's assuming you were coming from the south and you took PCH.
you never mentioned what time you left your house and what time you got to your destination. those key pieces of information are important...were you driving during rush hour(s)?