The laws specify what you can and what you cannot do. If the intent of the authority was that you were allowed to drive at 45 mph, you would have a speed limit of 45 mph, not a speed limit of 40 mph. If you go at 41 mph, you are breaking a law and can be punished.
That said, law enforcement officers usually have some leeway on how to enforce the law, and they could very well let it pass with just a warning (or even ignore it if they have more pressing issues); the circunstances of it are specific to every situation and officer.
The only point that could be made would be if the difference was so small that it could be argued that it can invalidate the evidence on the basis of margin of errors. If the radar catches you driving at 41 mph but the error margin of the radar is 5%, you could argue that you were driving at 39 mph and that the reading is due to the error in the radar1.
That would enable you to challenge the evidence (but here the point is not that you are allowed to drive at 41 mph but that there is no proof that you were driving at 41 mph). From what I know, most police forces will be aware of that and avoid issuing fines unless you are well above that margin of error2.
In fact, in Spain word of the street is that radars are set to account to possible margin of error of the radar, plus possible margin of error of the vehicle speedometer -even if it is the vehicle owner's responsibility to ensure that it works correctly
- and some leeway.
2Some people post on the internet the "magic formula" of how many % of speed you can go over the posted speed limit based on those calculations. Of course those magic formulas rely in the radar and the speedometer being 100% accurate and the driver never getting distracted a few seconds and passing it.
So, even assuming that those magic formulas are correct, if either the radar or the speedometer are not accurate or the driver gets distracted for a few seconds, you are at risk of getting a ticket.