Re: Lying Speedo
I imagine one major reason for the speed overestimation is that the speedo doesn't know how much tread is left on the tyres, and this can affect the rolling diameter by up to 10mm (2 x 5 mm wear) for worn tyres.
So basically I am surmising that your speedo will overestimate your speed more when your tyres are worn since I am sure that it must be conservatively calibrated for a new set of tyres.
Not too sure why they get much more inaccurate at higher speeds, I
would tentatively guess that there is some tyre slip even in normal road usage at higher speeds.