90% less smog-forming pollution... how did they calculate?

I don't know all the reasons, but here are a few:

That infamous gas-tank bladder prevents evaporation (as long as you never spill any gas). The car only burns about half as much gas as other cars (national fleet average?). The HSD allows the gas engine to always (or much more often) run at full design rpm, avoiding the higher pollution that occurs when the engine runs too slow or has to work too hard (not only can it run fast when the car is moving slowly*, but it can let the electric motor do the additional work under high-demand situations). The warm coolant stored in the thermos warms up the head before starting, greatly reducing the pollution of cold starts.

* In conventional cars, at very slow speeds the engine turns very slowly also. The Prius can run the engine at full speed, turning MG1 (?) as a generator to power MG2 (?) as a motor, to move the car. The inefficiency of the double conversion (mechanical to electrical to mechanical again) is more than offset by the efficiency of letting the engine turn at full speed. And as a bonus, the engine pollutes less at full speed. Finally, if this condition produces too much or too little power for the required acceleration, the battery can be recharged or drawn down as necessary.

Cool car!

 

The bladder in the fuel tank reduces evaporative emissions. The atkinson cycle engine produces fewer emissions. The coolant is stored in a thermos to reduce start-up emissions. The computer keeps the engine warm with start-ups if needed to eliminate the increase in emissions from running a cool engine. There are additional scrubbers in the exhaust system. The valve timing is variable and is adjusted relative to rpm and load to reduce emissions. Regenerative braking captures part of the kinetic energy to reduce engine demand. Engine does not run unless needed. (this does not mean that running on electric only is a good thing in the long run. Use the engine as it is the most efficient way to propel the vehicle.) Low drag coefficient reduces engine demand. Consumption screen encourages efficient use of engine, further reducing engine demand. The AC system employs the more efficient variable speed scroll compressor which ultimately reduces engine demand.

I know I missed some but as it has been sung, "These are a few of my favorite things".

 

Oh yeah Daniel, how could I forget the rpm thing. That is a very important part. In many ways the car sacrifices mileage for reduced emmisions. This is one of the ways because turning the engine at a constant rpm and putting excess production into the battery results in an overall loss in mileage, due to conversion inefficiency. Starting the engine to keep it warm costs a little mileage too.

I also forgot to mention that the CO2 emissions are not included in these figures and are still very high in the Prius, although lower than other cars due to higher miles per gallon

 

There are a ton of data about cars and emissions at http://www.driveclean.ca.gov/.

Their basic factsheet can also be found in the PriusChat file library.

Actually, being an AT-PZEV class vehicle, the '04 Prius is approximately 96% cleaner than the average 2003 model vehicle.