A few miles either way is easily the difference in vehicles, radars, speedometers, etc., but much more than that, and you're simply flat-out breaking the law. It's always 'wrong'. Do many people do it? Yes, but that doesn't make it any less wrong.

Typically speaking, the law makers choose the speed based on a set of conditions, and not on arbitrary things. For example, road conditions, safety, traffic amount, business or residential zones, school zones, surrounding, etc, all get included in what the safe operating speed of a vehicle is on that road.

Here in Texas, we have some highways with 80MPH speed limits on them (we share the fastest speed limits in the country with Utah). But these speed limits are on two major interstate highways, and our roads here in Texas are awesomely kept up. I drove through New Mexico and Arizona a few years back and was shocked at how bad the highways there were compared to Texas. The speed limits are that high because it's safe in those areas of long stretches with no towns nearby to drive at that speed in a passenger vehicle. Trucks (the big ones) are held to a lower speed.

So the better the road, the better the condition, and the farther away from dangerous things--both for driver and for people outside of the vehicle--speed limits are set for safety and to violate that is to put your life or someone else's in danger.

As such, I can only conclude that anything more than a few miles over is just wrong.