I have a '91 F-150 with a wonky speedometer. It seems that the gear in the transfer case is iffy, and under certain conditions, like lower throttle, the speed will drop off. I am sure the gear could be replaced in the transfer case, but that seems a lot more involved.

Has anyone tried to retrofit an aftermarket speedometer using the ABS tone ring? I think the '92+ trucks use the ABS tone ring for the speedometer in the first place. I was wondering if something like a Trailtech Vapor might be able to work if I wired it right up to the ABS tone ring. It normally uses a hall effect sensor -- isn't the tone sensor about the same?

Obviously, this would disable ABS, which already isn't working on my truck.

Thank you!

 

Something much easier would be to put a GPS speedometer on your dash.

 

You have already narrowed it down to the gear in the transfer case and not the cable, speedometer head, driven gear assembly or are you just guessing that is the problem?

I found the shaft that the driven gear attaches to was bent on my truck. I’m surprised the speedometer worked as good as it did.

 

I've run the speedometer cable with a drill and it works smoothly. Though the cable, or cluster, makes a high pitched whine some of the time, around 50mph+ (indicated).

That shaft could be bent on mine, perhaps.

I know a GPS speedometer is easier, and maybe I should do that. I'm just curious if someone has tried this as it seems like it *could* be fairly trivial. And nice to not worry about GPS reception.

 

Desires like this are usually resolved by the individual. Normal people would find a different normal answer.

There is a reason why there are so many points on the tone gear in the differential. Could you make a gear with the same amount of points that would fit inside the transfer case? Sure. Could you find a sensor that could 'count' those points? Sure. Could someone make a module that can interpret that signal? Sure. But there is no profitable company that would do that. Making stuff like that would fill a niche that nobody would fill.

The less points on that ring, the less sensitive the signal would be so low speed would be worthless.

Is GPS the way to go? That is what I would do. Would GPS work in all situations? No, but do you really need a speedometer 100% of the time? I think most people know when they are going faster than they should.

 

Question: Is the speedometer driven by a gear or tone ring in the transfer case?

A 1991 should have. a gear, that is accessible by removing the drive shaft, then the tail housing of the t-case. That's it, should be like 15 minutes to take out.

Am I wrong?

 

There is a gear. No, you are correct. The OP said that removing the tail housing on the transfer case is too difficult and would rather reinvent the wheel