ROG:
eric the judge:
When my car satnav reads 70 my speedo reads 77 and my car is only 5 years oldThey can be 10% out on over read
On the flat the satnav will give the true speed
I am surprised to hear of a 5 year old car that reads that much over. Would be interested to know the make. My wife bought a brand new Mini Countryman in 2015 and it was only 1 mph over.
Have checked a few including my own and have found none to be more that 3 mph.