BTW - for most Polish drivers, speed limits signs are perceived as a mild suggestion. :)
The way I see it is that a speed limit sign gives you a limit for 65 year old driver, driving a 1.2 litre engine car.
Now, to calculate YOUR speed limit based on the sign, you should substract your age from 65 and add
the result to the speed limit (one kilometre per hour for every year of age difference). Then, based on
your car's engine, you can also add 2 km/h for every 100cc over 1.2 litres. OK - that sounds a little bit
complicated, but really isn't. Let's take an example...
You are 28 years old, driving 1.6 litre car, and you see an "80 km/h" speed limit sign. The limit for you is:
80 + (65-28) = 117 (age bonus)
117 + 4*2 = 125 (engine bonus)
It is OK to go slightly over the limit (let's say 8-10 km/h) so you shouldn't really go faster than 135 km/h
on said road.