GPS is susceptible to atmospheric conditions, obstructed views (4 satellites minimum required for positioning) and a number of other factors, most of which are

GPS is susceptible to atmospheric conditions, obstructed views (4 satellites minimum required for positioning) and a number of other factors, most of which are undetectable at the device level. Car speedos are done with over-read deliberately as a safety measure. Better overestimating than under. As a driver you can make conscious decisions about what speed you're driving based on all evidence available, including an understanding of how over/under your speedometer returns results. With a GPS based speedometer you can't realistically do that, you can have no idea what the accuracy percentage is at any particular moment in time. It might be over, it might be under. No one can make good decisions based on junk data.