It's a pretty simple to use type of hall effect sensor that works on 5-24v, rated up to 125C, and detects ferrous metal:

http://m.littelfuse.com/~/media/elec...asheet.pdf.pdf

I have these on my crank & cam pickups too.

The ring came from an eBay / Amazon seller and was only $14. Mounting bracket is just some bent up some aluminum stock and fastened to the trans via an unused threaded hole. I strategically hacksawed the ring in half so I didn't have to dismount the CV axle. I just eyeballed the air gap as the angle isn't totally perpendicular and it worked the first time out.

A setup like this can supplement the Speedbox or perhaps interface to a less featureful speedo driver motor. But the Speedbox has some quality features and now Josh has taken the testing leap for all of us