Internalizing the environmental externalities of operating them.
> If they are already paying more for fuel, doesn’t that already “tax” them more?
Fuel taxes are a mechanism for the additional (that is, beyond road usage, in the context of the grandparent post) taxes on inefficient vehicles.
> It seems like such a tax is regressive — rich people can buy new efficient cars more easily than poor people.
That's actually a reason you might want a method other than exclusively fuel taxes to implement the additional taxation (which may be via, say, a means tested tax subsidy for purchase of more efficient cars, rather than directly another extra tax on inefficient ones), because fuel taxes are operations taxes but don't directly effect purchases (they may be taken into account, but the poor who feel they need a car will often buy what they can afford up front even if it is not fully rational because real people tend to fall short of rationality in some predictable ways, and discounting deferred costs is a big one.)
Taxation serves two purposes: (a) to fund roads, and (b) to discourage society damaging behavior. For (a), we need to tax all vehicles, for (b), we need to tax some vehicles more than others.
> It seems like such a tax is regressive — rich people can buy new efficient cars more easily than poor people.
True. Perhaps the efficiency tax can be built into the initial new car purchase while the use tax is ongoing (those old cars will eventually fall apart or fail an emissions test anyways and be taken off the road). Anyways, these things can be worked out.
Here's the Norwegian tax authorities form for calculating taxes. While not all the descriptive text is available in English it seems most of the form is:
https://www.skatteetaten.no/en/person/duties/cars-and-other-...
> But given CA's environmental goals, this would be counterproductive.
That is irrelevant, we are reaching a point where taxation on road miles driven vs. fueled used becomes unavoidable. What will wind up happening is that efficient and clean energy vehicles will still need to be taxed to account for usage, while less efficient vehicles will be taxed even more (to discourage use).