Modern Vespa

The speedo on my GTS 300 totally overreads. When I drive 60km/h, it reads 72, at 80 it reads 92 and at 100 it reads 112; thus a constant overreading of 12km/h. Is there any way or method how I can rectify this?

In order to be homologated in Europe, regulations require motor vehicle speedometers to:

a) not show less than the actual speed;
b) not show more than 10% + 4km more than the actual speed

So manufacturers have a legal interest in going a bit higher than the actual speed, because going under is not allowed, while there is a permitted margin for going over. They also have a marketing reason for going higher, since it gives customers the idea that the bike is faster than it really is -- the numbers you are citing are suspiciously just under the permitted maximum.

I have a third-party speedometer on my Sprint (I can't say the brand name or I'd have to spit) and it reliably matches the GPS-derived actual speed.

Just add some markings - I used white label-printing tape - to mark the important speeds. Note my speedo is an mph one.

They all have it, so has my BV300ie.
Don't worry about it. Just substract 10km/h and you're fine.