I put slightly bigger tires on my truck which eliminated most of the speedometer error. Instead of reading 5% high it's almost dead on, less than 1% high.
As has been alluded to , it depends on the tire size on the driving / power wheels . As to how accurate the speedo is . Never worried about the odometer being slightly off .
We had a used Olds that someone had put a wrecking yard differential in . Car was traveling much slower than the speedo was indicated . I changed tje little plastic speedo gear and got it pretty close . Quit worrying about it .
