Let me start with the fuel economy/emissions trade-off thought. When we had the X-Prize Sonata tested at Roush, we were given a 40% increase in fuel economy -- from stock 30 HWY to 42. All emissions were under EPA limitations. And in fact, all were below Hyundai's posted numbers as acceptable. The vehicle had about 40k miles on it when tested.
Some years ago I spoke with a chief engineer at a Canadian CNG retrofit manufacturing company. He told me that every January 1, all of their certifications became null and void. They had to find a clean vehicle for every chassis/engine/model year they wanted certified for that calendar year. Step 1 was to certify it as a clean slate, where it had to meet manufacturer's posted numbers for what emissions should be. Next step was to install their system and retest. He said that in all the years he had been doing that, they NEVER had a "clean" vehicle pass without massaging the daylights out of it.
With that said, in my opinion EPA and CARB (the 2 entities I've had to become familiar with) are 5% about pollution and 95% about making money. Case in point, if a CNG system was certified last January, the vehicles didn't change, the CNG system didn't change, why does EPA require another $10,000 per vehicle to recertify again next January?!?
Next point; as stated, not one of the vehicles this CNG company used for certification would pass in stock form. They would often buy them from a dealer and drive them straight to the test facility -- brand new! Sometimes they would try to get them with a couple thousand miles on them so at least the rings were seated. Didn't matter. Are vehicles as clean as statistics would suggest? If you improve
combustion efficiency, are you really increasing pollution?? We now have a rather high-end EMS 5003 5-gas analyzer we use when testing technologies. It's portable with a 12 volt power plug, and with the Lab View software, we can simulate the type of data in the Roush Report. It may not be certifiable, but at least we know if we're increasing or decreasing emissions.
Lastly, many of the technologies we advocate for automobiles we first tested on small engines. There is no computer, so we can control things ourselves. For some projects we have even computerized the small engine so we have digital signal processor accuracy over the controls.
Lots of folks try fuel economy tricks only to have them cause a Check Engine Light (CEL). Curiously, the same tricks sometimes work on other vehicles with no CEL. In reality, someone needs to have an understanding of their vehicle when modifying it. It will have its own personality. For the average person, it would be nice to know someone that is an expert in that field, that has a broad range of knowledge concerning fuel economy. It's a much shorter learning curve when you only need to learn what makes your own personal car tick.