speedo adjustment

find a nice strech of highway with mile markers, drive ten miles marking your start and end odo reading. divide your odo reading by 10 and this is the size percentage difference you need to make for your speedo gear in the trans.
For example:
if your odo read 11.0 miles and your gear was a 30 you would need to get a 33 gear. ie 30 x 1.1 = 33. if your odo read 9 the it would be .9 x 30 or 27 gear needed.

if you want it to be more accurate drive more miles, and check the mile markers with a car that has a validated odo.