It's astonishing the amount of misinformation that is being spread 'out there' in the Internet about the potential range of Part 15 FM, even by individuals and Forums that obviously should and do know better.
I've run across it on Boards frequented mainly by licensed broadcasters, and to some extent, that is understandable due to ignorance. On one, a participant stated, quite boldly, that any FM signal that went more than 30 feet was illegal, and the operator a pirate. Where he got that information, I have no idea, but I'm sure he read it somewhere.
On another Part 15 site, one which is well known to some of the participants here, it was recently matter of factly stated that any signal that went more than a 'couple of hundred' feet was illegal (obviously borrowing that statement from the FCC directive on Part 15 FM - they use 200 feet to an ordinary consumer radio as a guideline), and that those operators were pirates (or at the very least, wanna be ones).
Both of these statements, of course, are blatantly false - if you do the quite elementary math (which we've done here on this Forum multiple times), it is quite possible for a Part 15 FM signal to theoretically get 600-800 feet of range to a sensitive car radio, line of sight.
When you get statements of potential ranges that are not rigorous, and don't include the type of receiving equipment and its sensitivity (and selectivity), some measure of obstructions between the transmitting and receiving antennas, etc., then you can almost be guaranteed those statements are there for perception and effect, and not in any way shape or form based on science, math or reality. As an example, car radios are much more sensitive than the ordinary consumer radios used by the FCC as guidelines in determining legality.
I've proven that you can actually get those types of ranges by actual experience here in Canada (in terms of experience, I can only speak for myself). Multiply the above calculated ranges by 4 (allowing for the increased Canadian allowable FM field strength) and you get potential ranges of 2400-3200 feet. I managed around 3000 feet, true line of sight (my transmitter at the top of a building on the crest of a hill, overlooking the receiver in a car at the bottom). Ranges where there were obstructions in the way (trees, buildings, etc.) were, of course, much less, but that's not the point. My legal, the Canadian equivalent of Part 15 signal, generated by an Industry Canada certified Decade MS-100, tuned by the manufacturer to meet the 1000uv/m at 3 meter Canadian regulations, could be heard at ranges far exceeding that 'couple of hundred feet' (times 4) pronouncement.
I didn't hide, either. There were newspaper stories on the station in the local Bowen Island paper, as well as a major Vancouver daily, the Vancouver Province. And we were also mentioned on CBC Radio, Canada's national radio network (the equivalent of NPR). I actually found that the Canadian media in general, including broadcasters, were supportive of what we were trying to do, and there was certainly no mention of 'pirate' in any of the press we received.
I also know that Industry Canada visited my island shortly after I began broadcasting, and was measuring something in and around my coverage area. I never had any contact from them, so I can't say for certain it was my signal they were investigating, but since I had never before seen them, and never saw them again (in the 10+ years I was on the island), I think it's a pretty reasonable assumption that that was what they were there for. It could have been the publicity of the station that brought them there, but even in Canada, we do have a few grumpy individuals that want to force their own personal agendas on others, regardless of the facts.
In conclusion, reading these blanket range statements makes me wonder whether any of these guys making them, particularly those on other Part 15 sites, have actually tried Part 15 FM out. I suspect not.
And my general philosophy is simple. If you haven't actually tried it out, and have experience with it, then you should be very cautious about opening your mouth.