Adding a VSS output to a Mechanical Speedo (reed switch)
 

Just made a quick search and maybe this link is helpful to people who only have mechanical speedo and want VSS output to mpguino...
Adding a VSS output to a Mechanical Speedo (reed switch)

Edit: some more stuff from same forum:
http://www.speedview.co/forum/Forum-Tech-Info

Awesome find, I will need to do that when my unit arrives. I am guessing I won't need a converter board between the reed switch and the MPGuino, and that if I do, I can modify the source enough only need a simple one.

~Wild_Doogy

Edit:

I found that my speedo already has a reed switch so I just brought some wires off the back and hooked them up to my MPGuino. I am not sure where the error is, but I did notice an occasional error in my speed readout where my speed seemed to double, but that sounds like a double pulse.