> Why should less efficient vehicles be taxed more?
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.)
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.)