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Two different cars with the same mpg can have wildly different emissions. A fuel tax wouldn't discourage higher emissions, only lower mpg!

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Hence the part about rebates for efficient emissions -- the base fuel tax should cover the maximum economic impact.

True, but the super efficient ones must burn more cleanly, and realistically, they are trying to reduce CO2.

And that is all dependent on how much fuel you burn. NOx emissions as well will differ as well but for the most part, a 20 mpg car will be much dirtier than a 50 mpg car.

If this were true, motorcycles would be cleaner than every car out there. But tests have shown that - due to the reduced clean emissions tech in motorcycles - even though they consume the least fuel, they produce on the order of 10 to 1000x some of the nastier tail-pipe emissions.

The same is true for lawnmowers, leafblowers, and snowblowers.

This is not quite right, one of the big tensions in designing combustion systems is the trade off of thermal efficiency (increased at higher combustion temperatures) against NOx generation (also increased at higher combustion temperatures).

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