Why is it important to keep the rolling circumference the same?
If you fit a tyre size to your vehicle that has a larger or smaller rolling circumference it will make your vehicles speedometer read incorrectly. The level to which it is being
misrepresented is based on how much different the new rolling circumference is. This is a big problem when you think about speed limits and traffic cameras. You think you are
doing one speed when in fact you are doing another.
If the circumference is larger the speedometer will read lower than your speed, if the circumference is small is the speedometer will rear higher than your speed.