"I wonder if this is a common misperception in the general public?" - Unfortunately, yes it is the general idea. Several reasons for it.
First there have been too many get rich quick schemes that sold a bunch of junk that didn't work. That gave anyone that tries a bad reputation to overcome.
Second they do not realize that a car design is a series of compromises and trade-offs. They are never a perfect solution, just reasonably good for what the company thought they could get you to buy and make them the most money.
Third the biggest factor effecting MPG is the nut behind the wheel. If they put the blame on the car company then they don't have to take the blame for driving while not thinking about it.
Fourth most of them are too lazy to even keep good records. Lets face it, someone has to keep very diligent records under very tight controls to be able to see a 4-5% difference made by an aeromod as opposed to the random statistical noise due to weather, passenger load, construction season, gas brands, varied driving routes etc. To ask someone to do A-B-A testing is a bit much, unless he happens to be one of those loony folks that hanks out a sites like this.
