I have found them to be 10% over what the sat nav says across the range. When I go through the roadworks on the motorway it's a 50 limit with average speed cameras, my tomtom live sat nav pings up it's 50 then gives you your average speed as you are going through the roadworks.

If I set the cruise control to 55 the sat nav gives me my average along about 3 miles as exactly 50mph. Not had a ticket yet