Speedo 70
Tomtom 62
Which would you trust?
Mine is bang on!
Aim GPS speedo tastic!!
Mine has also been verified by the boys in blue! 
2006 “S” bang on …
If it reads 70 then i,m doin 70 …
Seans under reads by a factor of 2.2 …
IE… his reads 70 when radar reads 156 lool …
GPS is VERY accurate … as long as its got a fix on 3 + sattelites
Mine seems pretty bad too 
08S
06 s reads the same as tom tom
05 about 8mph more than Garmin
09
speedo 80
TomTom 73
Mines pretty much bang on. I think it was 2mph out at 100 (on track of course)
8mph out at 130!!! On a runway…
normally 5 to 7mph out of whack from speed to TomTom.
According to Walshy’s speed gun mine is 100% accurate.
Speedo 70mph
Tomtom 68mph
06S
quite accurate in my book 
Manufactures are allowed a tollerence of 0% to 10% on the high side, ie the Speedo must always read higher than the actual. Normally the Speedos are quite acurate and the diffence tends to be different size wheels on the same calibration set up. Only sure fire way to check is to count the posts along a motorway over a couple of miles at a constant speed.
Has always been within 3mph of the garmin.
Mine is generally within 1-2 mph (vs TomTom).
It can also vary a bit with tyre wear. A048’s would theoretically be 1.25% difference new vs borderline legal - R888’s maybe 2%?
Pretty accurate compared to other cars, wife’s MINI overreads by between 5-10%
I vMax’d my 2004 S2 Exige in Germany on the autobahn coming back from the Viking Furball in 2007 and can assure you that the speedo gets more inaccurate the faster you go. For example at 147.2MPH which was the absolute maximum registered on my TomTom One the speedo was pegged at 160MPH.