Speedometer is six mph too fast

Hello, just wanted you to know that I was able to get the dial to read fairly close to accurate (i.E. 1 to 3 miles faster than what my GPS unit reads) by taking off the plastic transparent shield in front of the speedometer then carefully removing the speedometer dial from its drive shaft. By lengthy trial and error (removing and reseating the dial onto its shaft) I was able to reduce the speed discrepancy from six miles to two-to-three miles per hour too fast. I was reading that the manufacturer habitually leaves the adjustment slightly too fast to prevent us drivers from claiming they caused us to get a speeding ticket. Then, on a hunch, I used my GPS unit to test my old 1984 Toyota Corolla that I have been driving for over fifteen years, and lo and behold, apparently all those years I had been using a speedometer that was four miles per hour too fast and just got used to the discrepancy over time. I have decided to leave the speedometer in the 1993 Corolla at the two-to-three miles per hour too fast setting since it appears I have living with the discrepancy for years already in my older car. However, do you recommend I get the 1993 car onto a dyno machine and then try to set the speedometer to a "dead on" reading? In your professional opinion, is it worth the trouble to make sure that the speedometer is fully accurate or as much so as possible? Could it be true that many speedometers are actually intentionally set to read slightly "optimistic" for the abovementioned reason? Thanks for any feedback you can offer.

Thank you very much for your latest reply stating that a speedometer cannot be expected to "be dead on forever". I had not even considered the prospect that external factors such as tire wear and road and wind conditions could affect the overall accuracy of the speedometer reading. I will not hesitate to continue to use your website as a valuable resource for automotive education as well as for advice on specific fixes. Thank you again.