I replaced my transmission (with mechanical speedo) with a newer one (electric sender).
So now I just have the mechanical speedo cable hanging there. I know marlin makes one to convert it from a mechanical to electric sender, but what if I need it the other way around??

 

Your beat I think. I am not sure if you can take the guts out of the mechanical one and put it in where the electric one is. I think the electric is in all chain driven cases and the mechanical is in the gear driven cases so ou may be out of luck. Change the dash.

 

Consult your local Toyota dealer today!!!
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95 Toyota 4-runner SR5 V6 3.0 Automatic 149,600 Miles (bone stock)

 

yes, you will have to change the instrument cluster as far as I know. Grab the wires from a junkyard and a cluster, I think you'll be far happier with the electric speedo anyways. When I do my tranny swap, I am going to a electric speedo to prevent my cable from chirping sometimes despite being lubed. also, the needle will be much steadier. Mine jiggles and wiggles, haha.

 

I seem to recall that it's just a matter of swapping the module on the tail shaft.
Can you grab it off of the old tranny?