If vacationing only in Frisco, not venturing out to other Pacific Coast cities, San Francisco car rental prices aren't even worth exploring. The car will get in the way. Especially since the city transport system takes into account all the needs of tourists, and daytime traffic jams and search for parking, especially in the city center, can spoil the rest.
Choose a cheap car rental in San Francisco or a more expensive option is worth only if you are going to take a trip to Napa Valley for a couple of bottles of famous Californian wine, go for a day "visiting" the stars of Hollywood, catch a wave in the town of Santa Cruz or go somewhere else outside San Francisco.
