Sorry, the joke got lost in the text medium! After some minutes I thought that I should have added a "/joke" an "/s", or something like that, but in the end forgot to do it.
But 40 mph is 64.36 km/h, isn't it? So perhaps it turns on under 60 km/h in Germany and under 64.36 km/h everywhere else? For some weird legal reasons?
It's 60 km/h everywhere else than the US, where it's 64.36 km/h for weird reasons.