My car is a 17 year old Vauxhall (old enough to drive itself? Who knows). Anyway I suspect the speedometer over-reads by a fairly hefty amount. This is because I can go past those 30 signs that light up if you're speeding at an indicated 35 and still not set them off. Not to mention people behind me getting very impatient with me if I obey the speed limit on my speedo. Is there any good way I can check how much it is over-reading, or even better, a way to recalibrate it?