It is very easy for speedometers not to under-report speed, given the 10% margin for over-reporting. It's not at all comparable to trying to maintain 100% uptim

It is very easy for speedometers not to under-report speed, given the 10% margin for over-reporting. It's not at all comparable to trying to maintain 100% uptime. Most of them deliberately over-report by about 8%. You can check this by comparing your car's indicated speed to the speed shown on your satnav.