You will be charged by California if you are a resident there for all the mileage you have driven.
IF you are retired & drive to visit others around the country, California will collect that tax per mile on top of the state gas taxes you paid in all the states you visited.
Seems like double jeopardy to me.
The pilot programs I have seen have not charged for out of state miles as long as it was documented, such as with a GPS device.
They have also repaid any fuel taxes paid during the pilot programs.
Only the mileage tax was charged, and only in-state miles were counted.
I’ve done a lot of math and it seems like a mileage tax benefits gas guzzlers. If you get 15mpg, you would pay more for a fuel tax than for a mileage tax. If you get 30mpg, you would pay less with the fuel tax, and more with the mileage tax.