Well, maybe you can answer my problem. The tranny I put in my car has a different drive gear in it. With my stock gears (2.80's) and the stock speedo gear, it read about 5-10mph fast. Now that I put 3.80's in it, I am guessing that a speedo gear designed to correct for 4.10's will make the proper correction. Do you agree?
All C-4's have a 7 tooth drive gear. You cannot change that, it's machined into the tailshaft. I've got 3.50's in my 77 Comet (C-4 also) and 25.5 inch tires and a 19 tooth gear is the correct one for it.
Mustangs Unlimited sells a 23 tooth gear, this is a non stock gear, Ford's gears go up to a 21 tooth count. There were 16, 17, 18, 19, 20 & 21 tooth Ford gears.
Be aware that the 23 tooth gear will wear out very quickly (you may get a few thousand miles out of them), so buy at least two. At least they're easy to change out.
Daniel