Originally Posted by freakasis
i know what the standards are, but i dont think any amount is acceptable.
A stock tire for my car is a 195/55-15, with a diameter of 23.5". With 10/32" of tread standard and 2/32" tread at end of life, we're already talking about a 1/2" diameter (3%) change during the course of its life just from wear. When should the speedo be accurate, with new tire, dead tires, or half way worn?
Then we get to account for the fact that pressure changes the rolling diameter of the tire, and we get further inaccuracy. Weight in the vehicle also changes the rolling diameter.
So, shy of GPS based speed detection, I don't think any of us have ever really had truly accurate speedometers.
And thats just the long way of me saying, "I really have no clue how to adjust the damn thing.".
Hope you find the answer you're looking for. Though, the "correct" answer to adjusting the speedo is to use the correctly sized tires.