Your speedo is allowed by law to over-read by 10% but never under-read. So the manufacturers tend to make them over-read.
Your satnav is probably more likely to show closer to your real speed.
Neither is likely to be 100% accurate, although the Satnav should be much more accurate most of the time – particularly on level ground.
Posted by Alec at 9:20 am, June 6, 2008