well, I have a digital schwinn speedo ($10.00 at wal mart) and it works great. the magneto doesn't interfere with it at all.
I originally had an el-cheap-o e-bay digital speedo, and the magneto defintily interfered with that one.
the only time it would work was when the engine wasn't running.
whatever you do, don't buy one of the old style analog speedo's off of e-bay ( they are labled as a retro speedo on e-bay). they look like the old style schwinn speedo's, but they are in Kmh, not mph.
the internal gears are all plastic (as well as the entire unit), and the gears end up getting chewed up.
I had one, and seriously, it lasted for about 3 days until the gears inside desitegrated.
these things were made for 20" bikes that are peddle powered only.
the gears can't handle 20+ mph for a long time.