Car speedometers are allowed a legal tolerance of +10% / -0%. This means that it is legally allowed to over-read up to 10% fast but is not allowed to under-read. Since they are calibrated from the transmission this allows for tire wear. New tires with a full rolling circumference will read a speed closer to true and worn tires with a smaller circumference will read faster than true. Also, speed cameras have a tolerance of about 10%. The tolerance is kinda like the manufacturers' indemnity to accurate speed display, allowing us muppets to fit 20in wheels over 14in and still have a legal and accurate within 10% speedo readout.
