recovering speedometer / odometer accuracy

Hi,
I just bought a 61 Econo pickup. It is 50yrs old now, so of course it has different wheel/tire combos on it(225/55R/16), I also know the gears in the rear have been changed to 3.50. The speedo/odometer are driven by a gear in the transmission that was sized based on rear end rations and overall tire heights. Has anyone figured out a method of "sizing" a speedo cable based on gear ratios and tire sizes, or is it trial an error approach? Also, is the speedometer gear generic and interchangeable between various species of Ford transmissions, or is there only a couple of tooth options available to us anyway? Thanks for any knowledge.
JT

Some speedometers can adjusted for accuracy....

use a gps to check actual speed and compare to what speedo reads. calculate percentage off. pull your speedo cable from side of tranny and count teeth (or color) and adjust up or down as needed. gears are available at summit racing or http://www.transmissioncenter.net/speedometer_calibration_______va.htm