Police radars are calibrated periodically. Those on commercial vehicles even moreso as trucking companies and taxis charge by the mile, and trade regulations require accurate measurement. The average car speedometer/odometer, on the other hand, is set “close enough” at the factory and often drifts from there. In the last 30 years, the adjustment on mine has drifted quite a bit.
As for tire sizes, while changing to a different tire size will affect the rolling diameter, and thus the accuracy of the speedometer/odometer, tires are like clothes or shoes in that even tires that say they are the same size aren’t always really the same size. Some 205/60R16s are wider than other 205/60R16s.
It’s worth noting that carmakers know that there will be some inaccuracies, especially if the tires on the drive wheels (the front for most modern cars) are changed, so they set the speedometer to read slightly high. With brand new factory original tires, pegging the speedometer at 55 MPH will probably happen at an actual speed of around 53 MPH.
In general though, you can count on it being 3–5% high with stock-sized tires, and up to 10% with non-stock tires (like my 175/65R13s instead of 175/70s) and/or a few years of wear on the drivetrain.