A $20k American-made electric pickup with no paint, no stereo, no screen

I think the best way is to tax fuel itself. This way worse mpg result in more tax.

Tax diesel more than gasoline, LNG less.

This is already done, in Europe most of the fuel costs are taxes.

That makes sense, but there would be no incentive to switch to an engine that emits less carbon for the same fuel consumption (if such a thing exists)

You don't create carbon out of thin air, it's from the fuel, so burning the same quantity of fuel will result in the same quantity of carbon, no matter how the engine works. Therefore a tax on fuel is a tax on carbon.

By definition, more carbon is less efficiency. Efficiency is about how much of the hydrocarbon you turn into heat. Diesels often burn a little dirty. That's partly because diesel engines don't burn all the fuel

Incomplete combustion is a big component of emissions, and it's exactly what you're saying doesn't exist

Those eventually degrade to CO2 so the increased warming from them compared to co2 by mass is temporary, like with methane.

Yes but since incomplete combustion is inverse correlated with fuel efficiency (unburned fuel is wasted fuel), it's not really a trade off. What is a trade off is NO emissions vs fuel efficiency. Burning your fuel oxygen rich will burn of more fuel, but also makes more NO (due to higher temperatures if I remember correctly).

We already do in the US (but the money mostly goes to road maintenance)

Ethanol blends get worse MPG, and entail additional carbon emissions in creation. They do not reduce carbon emissions.

What is the point of the link?

Unless you play in the nuclear physics, Carbon in is Carbon out. Carbon in fuel is Carbon out of the engine.

Isn't that what a carbon tax is? Adding a tax to the fossil fuel based on carbon content.

Thereby penalising existing vehicle owners who can’t switch to a more efficient vehicle overnight.

We have to come up with a rigorous alternative that doesn’t disproportionately affect lower income folk, because people tend not to be overly concerned about nebulous concepts like the climate impacts on unborn future generations, especially when my carbon impact at the margin is negligible when taken in context of global population.

If it is an issue - then option is to have less driving. Take a bus once in a while. Or bike.

Or switch to another old vehicle. Take old Golf instead of RAM, etc.

Apparently not enough, as USA has quite cheap fuel. Add 100% carbon tax and people will start to pay attention to MPG ratings. With x2 price increase gasoline in USA is still cheaper than in Germany.