> The o2 sensor is the thing that automatically leans the mixture when you
> drive at high altitude, right?  Or am I thinking of something else...

This is kind of a side effect, and probably not applicable anyway, since
the O2 sensor is usually not in use when driving up high (^;

You see, the system goes into an open-loop mode. bypassing the sensor at
something like 3000 rpm and above. Then engines make so little power at
altitude that you usually downshift and keep the rev's high.  The airflow
meter, which kinda measures air mass, kinda compensates.  I understand the
hot-wire versions do a better job.

In closed-loop operation (normal driving) the O2 sensor will do a good job
of controlling for optimum burn, including compensating for altitude.

malcolm