While just looking for something else I ran into a UN agreement signed off
on by vehicle manufacturing and exporting countries.  It established set
rules for speedo accuracy.  It was signed off on in February 2003.  It is an
updated addendum of an agreement from 1958 governing international vehicle
rules.

Agreed that under the defined rules for testing of a speedo (long list) it
shall NOT read less than actual speed.  Rules on how a speedo is marked
etc.
accuracy: speed actual must not be more than 2.5 mph over speed indicated at
the lowest listed test speed (30mph).  After that it gets strange. Now you
get that 2.5 mph plus 20 percent of the max speed of the speedo minus the
30mph.   That means up to 30 mph it could be 2.5mph over but not less than
30mph.  At higher speeds if the speedo was 110mph you could be as much as
2.5mph plus 20 % of 110-30.  Or about 18 mph off reading high.  An 85mph
speedo could be about 13mph high.

US and Canada rules though still lax are much tighter than that.  But
european vehicles shipped to the US need only meet the UN rules as the US
speedo rules are just a mfr agreement and not a law.
jimt