92 vs 100 seems like a very large and unacceptable variance.

Yr speedo accuracy will vary, as yr tread depth on yr tyres will vary up to 10mm between new and old. 10mm wear on an 18" wheel/tyres will affect yr speedo accuracy by around 3-4km/h new to old.

In the country, they often paint markers on the road 1km apart. if you can find one of these, drive beween them at say 100kph (indicated). This should take you 36 seconds. any variation to this 36 seconds will prove how accurate yr speedo (and yr tomtom) is

edit: lol leasaunce, you just deleted yr comment which makes mine obsolete