There is no general answer because it is different for every wheel / tire dimension, depending on the rolling circumference. The speedometer is designed so that it is within the norm with a permitted tire size. If you then put a permitted tire format on it with the smallest possible rolling circumference, it may well happen that the deviation is only 1%, which is also legitimate, because by law the speedometer may not show less than you actually drive. It can also display the exact speed that you are currently driving, there are only limitations with regard to the advance.
There is also another point that makes it difficult to make a general statement: The centrifugal force and its influence, especially on "soft" tires (mostly those with high sidewalls). In the case of such tires, the percentage deviation usually decreases the faster you drive. This has to do with the expansion, which ensures that the wheel rotates more slowly in relation to the driving speed, which at the same time reduces the speedometer advance. For this reason, by the way, the tires of vehicles that are very often driven very quickly wear more in the middle than on the flanks, because this is where the rubber bulges outwards.
