I've noticed with the proliferation of those irritating radar smiley speedo signs - the ones that give you speed readout and a if you are 30 or below and a if you are over 30 - that my speedo is reading fast.
It's not just one of these signs, but it's quite consistent. If I'm doing about 30 on my speedo, then the signs say 26-27, and today I approached one with 34 on the clock and it gave me a nice big smile with 30 mph.
Vish mine does the same I think all manufacturers make their speedo's read fast, I saw an article in some car mag (Max Power, Fast Car, etc., etc.) where they tested a few cars through timing lights and they all read fast then they put larger tyres on and run them through again and they were all pretty much accurate.
Generally speedometer accuracy is plus 10% so in theory it will never show your doing a lower speed than the actual. Accuracy is tested with the original equipment fit of tyres by the manufacturer (usually two tyre suppliers are used per type of vehicle). Tyre tollerances on rolling diameter can mmake a slight difference but can usually be ignored.
So 33to33 at 30mph
70to77 at 70mph
some speedo dials have small markings that indicate the settings used when calibrating the units.
I've seen 17mph on the 'Smiler' when I was virtually stationary, but that was after heavy braking, which probably confused it.
What narks me is the sour look they give you at a mere 31mph. Having said that, those things are infinitely preferable to a gatso.