1: https://en.wikipedia.org/wiki/Corporate_average_fuel_economy...
1. Poorer people tend to drive older vehicles, so if you solely encourage higher fuel economies by taxing carbon emissions, then the tax is (at least short-term) regressive.
2. You can work around #1 by applying incentives for manufacturers to make more efficient cars should lead any carbon tax
3. If you just reward companies based on fleet-average fuel economy without regard to vehicle size, then it would be rather bad for US car companies (who employ unionized workers) that historically make larger cars than Asian and European companies.
4. So the first thing done was to have a separate standard for passenger vehicles and light-trucks, but this resulted in minivans and SUVs being made in such a way as to get the light-truck rating
5. We then ended up with the size-based calculation we have today, but the formula is (IMO) overly punitive on small vehicles. Given that the formula was forward looking, it was almost certain to be wrong in one direction or the other, but it hasn't been updated.
Every single one of your ideas has problems that are solved by a carbon tax. Taxes are simple, they accomplish what you want, and they don't have loopholes. A carbon tax will _never_ have the unintended consequence of making emissions worse. Many of our current regulations, including the one I was responding to do exactly that because they actually cause people to buy larger trucks than they otherwise would with worse fuel efficiency.
A carbon tax might not on it's own be enough to solve the problem (especially if you set it to low), but no matter what level you set it, it will help. Thanks to unintended consequences, many of our current regulations are actively counter productive, while _also_ having negative economic and other costs.
Let's say that instead of taxing carbon, we pay people a bonus for emitting a below-average amount of carbon (proportional to the amount that they are below average by). If the amount is in a certain range, it will be too small an amount for wealthy people to care about, but large enough for poorer people to do things within their means (e.g. carpooling) to try to get it.
The results would hit certain geographic areas much worse than others, and (if priced enough to change behavior) would also probably depress car sales, which are two reasons why the federal fuel tax has been flat for over 30 years.
The original suggestion could be collected at point-of-sale for carbon emitting products. Gasoline, airplane tickets (based on average for the flights), even electricity are easy to measure and charge at the point of sale.
In your example, the person has to prove how much they didn’t emit, which is way harder in practice, to get the credit.
And similarly i would extrapolate to do we tax the buyer of electricity (which could be green sourced) or the manufacturer - the gas burner. Or maybe even at the first point of contact with the carbon source, the oil company.