I was reliably informed some years ago that the manufacturing tolerance on an analogue speedo was +/- 5%, BUT because car manufacturers didn't want to get sued they persuaded the speedo manufacturers to make the tolerance 0 -> +10%.
Of course, this has the added benefit that servicing comes round 5% (average) faster than it should (good for service dealers), plus recorded mileages are 5% more thereby devaluing you car more (good for sales dealers).
When I'm King I'm gonna put a stop to all this cr*p!
Sorry to hear about yours Zedrush. I got done a few years ago for 34 in a 30 so have lots of sympathy. Mine was an A road and although in a residential area, there is not on-street parking, wide carriageway, footpaths both side separated from the carriageway by grass verges - and one of the local councillors lives along there. His name's on my list too!
And whilst I'm king, all speed limits will be made advisory, with prosections for careless, without due car and attention, dangerous and over forms of cr*ppy driving. I'll even let them use camera evdince in a prosecution of these.
Mutter, mutter, mutter .....