Another way to do it, if you have the ability to turn the outputs on each transmission and observe the output of the speedo gear and assuming the old transmission was accurate with the speedometer, is as follows:
Make note of the output driveshaft position (example turn the yoke vertical) and use a pen to mark the speedometer output. Carefully turn the driveshaft through 10 complete rotations while counting the rotations of the speedo output of your new transmission. Estimate as closely as possible any partial turns of the speedometer output during the 10 rotations on the driveshaft. (example 10 driveshaft turns produced 5-7/8 (5.875) turns of the speedo output.) The more turns you do the more accurate you can estimate the ratio, just remember to use the same number of turns every time you count.
repeat this several times to determine an accurate ratio between the driveshaft and speedo output. Sometimes it helps to have a helper slowly turn the driveshaft through the 10 turns while you count the rotations of the speedometer output to simplify concentration on the task at hand.
Repeat this for your old transmission. (example 10 driveshaft turns produced 3-1/4 (3.25) turns of the speedo output.)
Have a ratio adapter made that has the ratio of the numbers you counted from the old trans divided by new trans. (example from above 3.25/5.875=0.553). This works if the only thing changed on the truck was the transmission. Your miles per driveline rotations will be the same, the only difference is the gearing in the new speedometer drive isn't calibrated to the same driveshaft rotations per mile. The ratio adapter effectively cancels out the error and makes your speedometer see the same driveshaft rotations per mile that your old transmission indicated.
