How fast am I going? 

For
a long time I caught myself mistaking the
tachometer for the speedometer.  One might think
that I could break this habit pretty easily. 
Why was this such a difficult habit to break? 
At first I thought that the problem was that
in one of our cars, the speedometer was on the
right (above) and on the other car, it was
on the left (below).  But as several readers
pointed out, the speedometer and tachometer
on the car below have the same numeric scale. 
So if a gauge shows 30, does it
indicate 30 miles per hour, or 3000 revolutions
per minute?  The gauge above doesn't have that
problem. 

Design suggestion

When
you have several similar displays close
together and lined up, people will confuse them
with each other.  This is especially true when
displays with numeric scales have the same increments. 
These displays should be made more distinctive by
making the numberic scale increments different.