I never knew so many people knew better than their speedometer. I imagine if you're driving a classic car, you might consider the accuracy of speedometers. I think you can be fairly confident in your speedometer otherwise.
"The law for car speedometers in the UK
The UK law is based on the EU standard, with some minor changes. A speedo must never show less than the actual speed, and must never show more than 110% of actual speed + 6.25mph. So if your true speed is 40mph, your speedo could legally be reading up to 50.25mph but never less than 40mph. Or to put it another way, if your speedo is reading 50mph, you won’t be doing more than 50mph but it’s possible you might actually only be travelling at 40mph.
To ensure that they comply with the law and make sure that their speedometers are never showing less than true speed under any foreseeable circumstances, car manufacturers will normally deliberately calibrate their speedos to read ‘high’ by a certain amount. As your satnav is not the designated device by which a car’s speed is measured, it does not need to incorporate any fudge factoring."
www.thecarexpert.co.uk/how-accurate-is-a-car-speedometer/
I am fairly confident that the defence of 'everybody knows speedometers aren't accurate' when charged with speeding wouldn't be terribly successful (with some exceptions where the speedometer was actually faulty)
I am also now only fully understanding the mentality of all the arsehole drivers I see on the road. I don't count myself as a particularly brilliant driver, but I'm a safe driver. I learnt late and I'm cautious. I stick to the speed limit (not over, not under). I have little desire to police other road users behaviour. I don't put myself in road situations that I'm uncomfortable with, if I can help it. If it weren't for the fact that their actions put other road users at risk, and can make a drive unnecessarily stressful for me, I'd be happy to say go forth and pursue your death wish. Some friends were involved in a pretty horrific collision when a tailgating, undertaking driver took a dangerous overtake on a dual to single lane carriageway. He was obviously frustrated with having to go the speed limit. Our friends survived with minor injuries. The passenger of the overtaking vehicle, and all the occupants of the vehicle in the opposite lane were not so lucky.
I don't get rationalising this behaviour. Tailgating and flashing is part of a pattern of driving which is just dangerous. It doesn't matter what your speedometer says, it doesn't matter what other road users 'should' be doing, it doesn't matter if you're an 'excellent' driver. Just chill and shake your head at the weirdos on the road. Enjoy your drive. Flashing and tailgating is as much 'policing' behaviour as deliberately slowing down is.