They Are As Accurate As The Old Ones The TomTom Ones Are Inaccurate As These Work On GPS And The Satellites Are Constantly Basically Trying To Catch You Up And Good Luck In Newquay
TomTom (or any GPS device that shows speed) should be accurate to well within 1mph on a straight and level road. A guy on one of the TomTom forums with access to proper speed calibration equipment confirmed this once.
It'll struggle on a bendy road, and going up/downhill will cause triangulation errors that'll make it underread on your actual road speed. If my TomTom's anything to go by, you'll also get the occasional glitch where it shows silly speeds.
The satellite system does have deliberate errors in the data, but the 'drift' should remain consistent over a short period of time. This makes GPS very accurate for calculating relative distances over short periods of time. Factor in the time and you have a very accurate speed reading.
A wheel driven speedo can never be considered 100% accurate. This is because the rolling radius of the wheels is not a constant. It varies with tyre wear and pressure, the latter of which varies throughout the day with tyre temperature.
Add to this the fact that manufacturers often sell cars with different sized wheels (e.g. 16" as standard with the option of 17". The 17" ones will have lower profile tyres, but the rolling radius still won't be identical to the 16"ers.) As long as the speedo shows speed within the accepted range of accuracy (-0% to +10%) that's fine.
For what it's worth, the digital cruse control display on the Laguna overreads against GPS by 2-3mph. This in turn is about 1-2mph slower than the speed shown by the needle.