There's a really cool speedo on the bay right now. If I didn't have one already, I'd be on it like stink on a monkey. Item number 220534381327 , and no, it's not mine. Just trying to help a fellow MB'er find a speedometer.
Yes, the cheap one, $20 one that I got from Pirates cycles. Before I even started I can see that the axles comes right to the end of the nut, which means the axle is just long enough for just the wheel and nothing else. I guess I have a cheap speedo for sale really cheap now. Maybe I will just leave it on there to look cool and not install the cable.
You can also try turning out the left side inner nut that is on the inside of your fork, then taking up the slack on the right side inner nut, this might give you enough room to install speedo, that is if you have enough thread to work with. I was able to gain about a 1/4 inch on right side by doing this. I also got lucky and scored a retro speedo made by Stewart and Werner in the 70s much more accurate and needle doesnt fly around all over. Them cheapos are not designed for high speed and vibration. Good luck, hey they look cool when your parked.