When the FAA certified general aviation aircraft years ago under CAR3, single engine aircraft were, in my opinion, given rather short shrift.
Much of the information contained in the POH of these earlier aircraft was of a rather general nature, and as been pointed out by others, the performance data was obtained with a brand new aeroplane, with an experienced test pilot at the controls.
Most twin engine aircraft, however, had an Airplane Flight Manual, and with these aircraft, much more detailed performance testing was done, so that the performance information contained in their AFM's was of a better quality.
Newer aircraft, certified under 14CFR23, have a much better POH/AFM than the earlier types, and even with newer single engine aeroplanes, generally speaking, the performance data can be relied upon, while at the same time, using that one commodity that most general aviation pilots should have....a large dose of common sense.
And yet, as I watch some newer 172's taxi past my private aeroplane at the airport, there they go, with 4 good sized adults and much baggage stuffed inside, and no doubt with the fuel tanks full.
Clearly, with some pilots, common sense, never mind the actual performance data...is out to lunch.
