I think the 5-10 mph above the limit that most people drive originates from the fact that police in most U.S. jurisdictions will almost never ticket someone going less than 10mph over the limit (except perhaps in school zones or construction zones). Due to that, the unofficial limit over time naturally drifts up to just under where you're likely to be ticketed.
This unofficial leeway likely developed due to things like mechanical issues with speedometers and tires causing reasonable doubt about actual speed within a few MPH. If they were to raise the official speed limit by 5-10MPH, then people would just do 5-10MPH above that. If police then started enforcing much more strictly, you're going to jam up the courts with more people contesting a 1mph over ticket as being due to speedometer calibration or whatever. Or just in general being much more resentful of the police for being so draconian.
This unofficial leeway likely developed due to things like mechanical issues with speedometers and tires causing reasonable doubt about actual speed within a few MPH. If they were to raise the official speed limit by 5-10MPH, then people would just do 5-10MPH above that. If police then started enforcing much more strictly, you're going to jam up the courts with more people contesting a 1mph over ticket as being due to speedometer calibration or whatever. Or just in general being much more resentful of the police for being so draconian.