Sat Nav Speed - Big John
As expected my Sat Nav (CoPilot running on IPAQ) indicates a different speed compared to my Octavia's speedo.

Sat Nav 28.5MPH Speedo 30MPH
Sat Nav 38.0MPH Speedo 40MPH
Sat Nav 67MPH Speedo 70MPH

Can I presume the Sat Nav is more accurate?

PS on travelling in a Seat Ibiza 1.9SDI
Sat Nav 63.5MPH Speedo 70MPH!
Sat Nav Speed - Marcos{P}
My speedo seems to over read compared to my sat-nav by 1.5%.
After reading several posts on this site I have come to the conclusion that my speedo is over reading and the sat-nav is probably more accurate.
I think speedos tend to over read due to bieng set that way by the manufacturers.
Sat Nav Speed - smokie
I think that speedos are NOT ALLOWED to under read by are allowed to over read by up to 10%. The manufacturers therefore err on the side of safety and most (all?) speedos over read.

A policeman once calibrated my Senator speedo for me using a radar gun, and gave me a certificate to prove it. It was exactly 10% over (at 112 mph), but I gather the variance can vary (as they do) at various speeds.

Sat Nav OUGHT to be more accurate, but I've noticed that when I vary my speed the SatNav doesn't respond immediately.

Very sorry for repetition of vary, and variances of vary.
Sat Nav Speed - Altea Ego
Yes as said its very very likely your speedo will over read by design. The sat nav will be more accurate, accuracy being greater the more satelites you have lock on.
Sat nav will respond less quickly to speed changes as it requires time and distance traveled to work, rather than wheel rotation (ie 6 revs of a wheel will give you a speed on the speedo. sat nav needs up to about 30-50 yards. (depends on satelite locks)

You now have the best of both worlds, you know - with certainty -the accuracy of your speedo.

Value my car