Tires of the same "Size", but from different manufacturers, can differ by 3% or more, even when new. Tire wear reduces diameter and makes speedometer and odometer optimistic. Speedo & odometer used to be driven by gears off the tranny, so the chosen gear ratio was always closest whole tooth count to the desired ratio and often was not optimized for all tire options. Today's speedos and odometers receive input from antilock brake "Tone Wheels" or similar sensors in the trans, but may not be any more accurate than gear-driven systems of old.AHTOXA wrote:Up to 10% margin of error is considered normal for car manufacturers and is not likely to be replaced as defectve. Many cars suffer from that and most owners do not even know about it.
Anyone who really has to know the vehicle speed down to a gnat's rectum uses a "5th wheel" or even an optical sensor.