speedometer driven gears
The "driven" gear is the one on the speedometer cable that plugs into the transmission or transfer case. Here are the types of driven gears Ford shows, but the 1980 - 86 trucks take Type 2 & 2a:
Fortunately Ford color-coded the gears:
Ok, so now we know what it looks like, and how many teeth and what color each part number is, but what part number do we need? Here are the tables to let you determine that based on your rear axle ratio and tire size. However, while this page was intended for to be for the driven gears, the tables also show the drive gears. Also, the tire sizes listed in these tables are the old-style sizes, so you will have to translate.