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[Tombola]

I think you will find that the ‘speed bongs’ are triggered by the Jazz front facing camera, not GPS.
I have a village local to me, normally subject to a 30mph limit.
However the residents raised a petition some time ago, seeking a reduction in the speed limit to 20mph.
In order to press the point, most their ‘dustbins’ (refuse collection bins) have had the number 20 painted on them.
Consequently, whenever I drive through the village on ‘dustbin’ day, the ‘bongs’ are triggered whenever I exceed 20mph.

You're talking rubbish

[Lord Voltermore]

I think it uses both GPS data and  sign recognition.  My driveway at home is only about 10 metres long,yet the 'Here' satnav  displays  it as an off road 'track' with no speed limit.(a grey roundel with two dashes)   Soon after I join the public highway  (which is a 30 mph limit)  it displays a 30 mph roundel and would chime if I exceeded it.   I am fairly sure this is before I have passed any 30 mph roundel that it could have read.  . So the system either uses  gps  data ,or somehow remembers the limit was 30 mph when I previously left that road. .  I think if someone put a 20 mph roundel on their dustbin it would read that and display 20 mph instead .  I dont know what would happen if someone falsely displayed a convincing roundel higher than the applicable limit. 

The newer mk4's  where speed alerts are compulsory  have "Here" satnav, not Garmin.  I think I read on this site that its more compatible with the alert system.   It does make sense to use GPS at times as many Urban areas in the  UK and  on the continent rely more on the drivers knowing they are within an urban speed area because of village name boards, presence of lamp posts etc   rather than displaying numerous speed limit signs all over town.

[Kremmen]

Does Here mapping have built in speed the same as OSM maps do

AFAIK 30 signs are few and far between because the street lights rule comes into play

[Lord Voltermore]

Does Here mapping have built in speed the same as OSM maps do

AFAIK 30 signs are few and far between because the street lights rule comes into play

It only displays speed roundels on the  driver display, Not also on the map display as the Honda Garmin did. Eagle eyed  front seat passengers can still see it     Saves taking  your eyes too far from the road.   It doesnt show your current speed but neither did the Honda Garmin.   (My old standalone Garmin does)