A few people have asked me how to correct the reading of their speedometer due to tire size changes.
To make a correction, the very first thing that has to be done is setting the "Use VID" parameter to disabled. If this is not done, the changes you make to the "Tire Revs per Distance" parameter will not make a difference.
If you have aftermarket tires and are lucky then you may be able to find correct value to use for Tire Revs/Mile off of the tire manufacturer's website.
If you are unable to find the information then you can use a GPS to measure your speed and compare to the speed the PCM is reporting to come up with a correction factor.
Example: Your speedometer reports 55 MPH while the GPS reports 60 MPH. You would take 55 divided by 60 and get a correction factor of .916. You would then take your current value for "Tire Revs per Distance" and multiply it by this correction factor to come up with the new value.
This usually is enough to get it pretty spot on, but you can repeat the process a second time to get it a little closer.