Anyone that thinks a Crown Vis will get good gas mileage hasn't driven one.
As for the whacker end of it, it's a gray area. While Ford, Chevy and DCX sell civilian versions of their current police package cars (Crown Vic, Impala and Magnum/Charger) it shouldn't be a surprise to anyone here that if you own one of the above cars and equip it, the public will think it's an official car based solely on their memory and perception of what a police car looks like. If you buy a retired CVPI or 9C1 that has spotlights, pushbars etc then it really shouldn't surprise you that people still think it's a cop car.
Anyone who buys a retired late-model squad and leaves all the stuff in/on it is generally being a whacker to some extent, because they know in the back of their head that an average person will think they are a cop. I don't know about the rest of you, but the last thing I want people to think is I'm driving a cop car.
Long story short: If you want to blend in and install your equipment without drawing unwanted attention to yourself, do yourself a favor and don't buy a retired late-model police car. They stick out like sore thumbs, as the poster "njt462" can attest to. The cops know it's not an in-service car, so they are harassing him. In his case, it still looked like an in-service MSHP upper mangement car (Lt. and above drive unmarked CVPI's) and every cop in the state knows it. Is it right for the cops to be harassing him? No, it isn't because -legally- there isn't anything wrong with the car (at least from what's been posted in this thread). However, it looks like an in-service MSHP car and the general public in the state of Missouri would think he's a trooper. That creates a safety issue for njt462 and the public, because if a member of the public comes running up to him stopped at a stoplight with a bonafide emergency thinking he's a trooper, that is a big problem.
I've seen very nice installs on civvie versions of the cars listed above that included a lot of equipment and didn't look a bit like a cop car. It's doable, but it takes $$ and effort to stand out from the whacker crowd.