Don't bet on that. DMV will look at the purchase date and insist on collecting the sales tax before they let you register the vehicle. If it's not CA smog certified, you'll likely have to get that done too.
Cities around San Diego have city sales tax in addition to state sales tax. The El Cajon Harley shop was compelled to collect city sales tax for people who lived in San Diego vs El Cajon as a home address.
When Borders book store was still in business, I preferred to shop at the store with the lowest city sales tax on top of the state sales tax. That was usually a good approach except that the store near Rancho Penasquitos had a much better computer science section than stores in other parts of the county.
I’m sure that a car can’t be registered without paying the sales tax. But... the whole sales tax is probably going to go to the state rather than the municipality getting a cut.