alan:

is that 12 mpg for normal 55 mph highway driving, at sea level, or
something else?

assuming newish ignition parts and engine already broken in, and no vacuum
leaks, advance ignition timing until she just pings going up steep hills,
then back off a few degrees.

not familiar with '81, but if O2 sensor involved, an old unit could easily
degrade gas mileage.

dan