Or in a ZX4? The silly little up arrow is wrong far more often than it's
right, or so it seems to the sense of feel.

The problem with installing a tach in these cars is the type of ignition
system. Since it has what is called a "double-spark" arrangement, you need a
tach that has a setting for two cylinders. The only add-on tach I could find
with that setting was a very expensive 14000 rpm motorcycle racing tach. So
what I did was get a recycled cluster and stick it it. They're not expensive
or hard to find, and fairly easy to install, but the problem is the odometer
reading will be off.