Re 90 days or 7501 miles to bring into California
This post addresses only the sales tax portion of your question, not the issue about whether you can register a non-California-smog-compliant vehicle in California when moving in from another state.
Sales tax issue:
You need to look at Section 6396 of the California Sales and Use Tax Law, and Regulation 1566 of the California Board of Equalization Sales and Use Tax Regulations, section (d) "Out of State Purchases of Vehicles -- 90-Day Test. " You can find these at your local county law library, or if you have access to Lexis/Nexis or WestLaw, or you can consult an attorney.
Regulation 1566, Section (d) provides:
"(1) For the purpose of determining whether a vehicle which is purchased outside California is purchased for use in this state, it is presumed that the vehicle was purchased for use here if it enters California within 90 days after its purchase. This presumption may be rebutted by contrary evidence satisfactory to the board showing that the purchaser did not intend to use the vehicle in this state.
"(2) Prior out-of-state use not exceeding 90 days from the date of purchase to the date of entry into California is of a temporary nature and is not proof of an intent that the vehicle was purchased for use elsewhere. Prior out-of-state use in excess of 90 days from the date of purchase to the date of entry into California, exclusive of any time of shipment to California or time of storage for shipment to California, will be accepted as proof of an intent that the vehicle was not purchased for use in California. Accordingly, when a vehicle is purchased in a foreign country or in another state and is later shipped to California, the period of use for purposes of the 90-day test will be measured by the interval from the time the purchaser takes possession at the out-of-state point to the time when the vehicle is delivered to a shipping agent or placed in storage for shipment to California.
"This subsection applies with respect to vehicles ordered on or after October 1, 1970. "
This is what the big trucking companies do. They purchase a vehicle from a California dealer, but then take delivery of the vehicle outside of California and put it into service for at least 90 days before bringing it to California for further use. You can find a good discussion of how they do this in the case of Engs Motor Truck Co. vs. California State Board of Equalization, (1987) 189 Cal. App. 3d 1458, 235 Cal. Rptr. 117, which you can also find at your county law library. The law libraries are usually within a block or so of the main courthouse in your county.
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