Originally Posted by stats007
By law all speedometers have to overread between 0 (wholly accurate) & 10% - they are illegal if they underread. Most manufacturers it seems average on a 3% overread which may fluctuate in either direction as speed increases. Tolerances on components are +/- so increased revolutions can have an effect in both ways.
I remember reading an article about an Audi owner that discovered his speedo was on the limit at 10% overread which theoretically meant at 50k miles he'd only done 45k and therefore the overread was devaluing his car. Audi weren't that interested IIRC.
The mileometer shouldn't over-read as that is an exact measurement and is a simple ratio issue, i.e for each wheel revolution the mileometer gears turn an exact amount. The speedo over reads as it has to account for the spring pressure against the needle.
Mechanical speedos use a magnetic flux to drive the needls boss so are not accurate, hence are set to over read.