All the four speeds are relatively the same to get at the speedo drive gear in the transmission, T-18, T-19, and NP-435.

It's color coded depending on how any teeth the gear has. Either 6, 7, 8, or 9 depending on the rear end ratio, and the cut to the teeth, either left or right handed. This is different to the speedo cable gear on the end of the cable that is also color coded with teeth rangeing from 16 to 21 teeth and is to be matched with tire size.

You need to know the current rear end ratio of the truck, and/or the color of the gear inside the transmission to get the right one, or to get the right match.

If your speedometer is off, it would be a good time to replace the speedometer cable gear with the right set of teeth too.

Hope this helps.