Your mileage doesn't sound that bad. Were you using cruise control/air conditioning?

If you took I-5 and drove through the grapevine before Castaic Lake at 70MPH heading south, that would have caused a major MPG hit. If you took the windy roads of SR-101, I'm surprised your mileage wasn't worse.

If you stuck to 65MPH and increased your cold tire pressure to the high 30s, you would have probably been closer to 48-50mpg. But I know that Bay Area-L.A. drive pretty well. I drive it every few months. Its tough to stay at 65 with everyone else driving 80-90.