While visiting Darren Decker's house Lexington, NC I had a chance to look at Brandon Barber's speedometer issue. We have resolved it since then, but here is the information I posted on Google Groups regarding how the system works.
Here is a quick video explaining how the speedometer system in a Delorean works.
First his speedometer dust cap is missing. He will need to order a new one from a vendor such as DMCMW or DPI. I don't think John Hervey sells this part.
Next we discovered that his angle drive does not work. I verified this by disconnecting it from the lower cable and turning the angle drive by hand. The angle drive did not turn on both sides when I turned it by hand. Something is broke inside of it and the angle drive will need to be replaced. I highly recommend the one John Hervey is selling.
DMCMW-DMCH Angle Drive (I've gone through several of these)
John Hervey -I've probably put 20k miles on mine. I'm not really keeping track but I installed it back in 2011. I drove 15,000 miles back in 2013 without any speedometer issues other than a dust cap.
Keep in mind the nut will need to be transferred from your old angle drive to the new one. It is a big pain to do if you have never done something like this before. I don't mind transferring the nut myself but it can be worth spending a few extra dollars for the convenience of having an angle drive that is ready to go.
The next mind boggling thing we discovered is the inner cable in his lower speedometer cable is completely missing. At a minimum he will need to replace the inner cable. I would recommend replacing the entire cable assembly (lower and upper) with a longer cable. This will allow him to bypass the counter box (there is a fancy word for it but I don't know how to spell it) and will make his speedometer less likely to fail in the future.
I got my longer speedometer cable from John Hervey and am very happy with it. I believe PJ Grady sells the same thing. I'm not sure if Josh Bengston has it or not. Bill Robertson claims you can buy a $9 cable at AutoZone and transfer the ends from your old cables to the new one. This would be an interesting way to save money.
Anyway, here is a link to John Hervey's longer speedometer cable.
Hopefully this information helps someone. As you can see there are a lot of components to go wrong in a Delorean speedometer system. Fortunately, it is very simple to diagnose speedometer problems.