Here's a copy of my post and the reply from about a year ago:
My out-of-warranty 2002 XG350's speedometer quit working on the way home
from work the other day.  All of a sudden it was reading 0 mph.  The tach
was still working, and the car seemed to be running fine.  After noticing
this, I was just about to engage the cruise control to see if that was
working or not, when the speedometer jumped back to life and has been
working ever since.  Anyone seen this behavior before, or know what to do
about it?

Reply from HyundaiTech:

Usually, the problem is with the vehicle speed sensor or the gear attached
to it.  It's nearly impossible to tell which is faulty without trial and
error, so I usually just replace both at the same time-- they're part of
the same assembly.

I bought the speed sensor and gear from the Hyundai dealer and then ran into
trouble installing them.  The old one is in really tight and, not knowing
exactly how things fit together, I sure didn't want to break the speed
sensor off in the tranny, so I went back to the dealer to have it installed.
They wanted to charge me $35 to "diagnose" whether it was the speed sensor
or not, but I insisted they just install the sensor.  It worked, and I
haven't had any trouble from it since.

Dan