[BDDkDvgZ]

Little late to the discussion.

There is margin of error as not all speedometers in cars are exactly dead on. Low tire pressure is enough to cause your speedo to become ~1mph off at around 55-65mph. Law enforcement must consider this when issuing speeding tickets.

I know for a fact the manufacturer of my car did not properly match the tires to the ratio's they used for the speedometer. According to the manufacturer, the tires needed on my car to replace the OEM tires on 17" rims is 215/40R17. But in order for my speedometer to be close to what it should be, I need 215/45R17's which are slightly larger tire diameter.