Hello, all. So, by searching, I know that the speedometer gear topic has been beaten to death here on FTE, but I am still stumped as to my issue.

Here is the background: I installed 3.55s in my 1990 f350. It has the bw1356, and when it had 4.10s the speedometer worked accurately. The stock driven gear was a 19 tooth, so naturally I installed the 16 tooth. I am aware there are different types of speedometer gears as shared on multiple threads, so ordered a gear from the dealership. It installed correctly, and appeared to have the same profile.

However, when I installed it, it was jumpy and didn't correct my speed to what I calculated. So, then I pulled the gear and found that it had worn as pictured. I reinstalled my 19 tooth and it is still working correctly, but obviously inaccurately.

What could cause the 16 tooth to wear like this within 100 miles and the 19 tooth to be fine?

My thoughts are that the driving gear is incompatible with the 16 tooth and the gears aren't meshing, but I have not found anything about the 16 tooth being incompatible with any of the nylon driving gears. Can anyone confirm?

Perhaps did the dealership do me wrong and give me a gear that isn't the correct diameter? It did not seem to be different. I can confirm with calipers later.

Another thought is that the driving gear is worn, but if that were so, then wouldn't the 19 tooth be working improperly?

Let me know what your thoughts are.

 

Is yours manual or automatic?
Not sure if they are different.
Possibly need a different drive gear.

 

It's 4WD, therefore the speedometer gear is in the transfer case. So regardless of transmission type the gear set would be the same, correct?

 

I think the auto vs manual 2WD are different. Maybe they gave you the wrong one.
Is the new gear exactly same shape and length as the old one?

Very what color drive gear is in the transfer case.