Is my Volvo XC60's TomTom more accurate at measuring speed than the cruise control?

When the cruise control on my Volvo XC70 is set at 60mph the TomTom says I am doing 58/59. When setting the cruise control to 61mph TomTom says I am doing 60. Which is likely to be more accurate, please? I am towing a caravan, so 60 is of some interest.

Asked on 10 August 2013 by NT, via email

Answered by Honest John
At steady speeds on flat open roads your TomTom will be 100 per cent accurate and your speedo/cruise control setting will over-read by between 2 per cent and 10 per cent.
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