Im confused now 
Did i not say HGV's were limited to 56mph?
Thats not to say drivers abide by the law regarding tacho's and limiters...magnets etc 
I still cant see how you see 90% of trucks travelling at 60+mph?
As 60mph is the limit on a motorway for these trucks.
Their tachometers will warn them once they go above 56mph but in the uk they can go to 60 without breaking the law.
There are differences in tachos I know, depends on the set up of the tacho but 60 is legal for hgv / 7.5t+
Dont know where you are getting this info from to be honest but any goods vehicle over 3.5 ton has a tacho fitted,which means it has a speed limiter fitted also,which is set to 56mph.It will not warn the driver that they are going above 56mph it simply stops the vehicle accelerating anymore.You can sit with your foot to the floor all day long but it wont go any faster than 56mph. Fact!
If it is going faster it will be either 1 of 2 things
1:- Overspeed, which is also known as over run- i.e momentum speeding the vehicle up going downhill
2:- Tachograph unit having been tampered with illegally
It makes no odds whatsoever what the speed limit is, the vehicle is only permitted to travel at the speed set by the limiter which is decided by the European Union in Brussels 56MPH
Any vehicle over 9 passenger seats is fitted with a tacho and the same rules apply, only difference is a coach is limited to 62mph