> Why should less efficient vehicles be taxed more?
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.
Many countries (Norway would be an example) have special taxes on cars that takes into account a variety of metrics around pollution, weight and engine capacity, as well as price. Overall car taxes in Norway are among the highest in the world, to the extent that it massively skews which car models people buy.
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:
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.