So I'm just getting all the things together, to do a gear ratio swap on my '87 F150, I'll be going from 3.55 to 4.56, does anybody know which Speedometer "Driven" gear (the one on the end of the speedometer cable) would be the closest to accurate for this gear ratio?

I'm hoping that I don't have to drop the transfer case, and change the drive gear on the output shaft, but I will if I have to. I don't need the speedometer to be dead nuts accurate, but if it's somewhat close to accurate, that would be nice.

 

I believe the easiest path here is seeing which gear you currently have then use that as a reference on what to change to. Or starting from scratch use the formula listed in the URL below. If you have the driven gear tooth count you can derive what the drive gear tooth count is to see if the available driven gears will get you in the ballpark. If not, then you know the drive needs changed as well.

TCI Speedometer Gear Info

 

Cool, thanks, I'll check it out.