The reason that I said I'd like to see where it's written into statute is that I can't think of a reason why it would be legal. It sounds like driver folklore rather than law.
Having a speed limit that can be exceeded simply for overtaking (rather than by emergency services) means the practical speed limit then becomes "the limit + 5" in traffic, which increases the chance for collisions (for no reason I can think of).
I'm of course open to being shown otherwise! I couldn't find anything searching for "Texas passing speed limit" or variations, but my license is European, so I'm not sure where to look.
