I did a search on corral and came up with this reply:
Its not hard to change the Speedo gear on the outout shaft. All you do is undo the rear case, and drain the tranny fluid out, then take it off and its right there, should be black, if you have 3.08's. You get the white one from any ford dealership. Just tell them you need a White 6 tooth speedo gear for the output shaft on a T-5. Or you can look in the FMS Catalog in the Rear End section. When done, clean the case as good as you can, then black silicon the case, and connect it and put the black speedo gear in place the connects to the harness, and bada bing bada boom yo done.
I hope to attempt the swap this coming Sunday afternoon or Monday.