The Brake Specific Fuel Consumption,
or BSFC maps (graphs) have long shown that
there is an "island" of good fuel economy
rising out of a "sea" of poorer fuel consumption.
The horizontal ( East West)
direction of a BSFC map
is almost always RPM,
but sometimes is average piston speed ( more "universal" because engines have different crankshaft strokes)
The vertical ( North South)
direction of such a fuel economy map
can be many measurements such as
Torque, manifold vacuum, throttle position,
or the more "universal" BMEP: brake mean effective pressure.
If Chrysler only put a single light out on top the driver's side fender
indicating a level of manifold vacuum,
didn't this only do half the required job?
To be most effective,
there should have been a second light, or second colored light bulb inside the same glass,
to indicate the driver was also inside the correct RPM range,
which is set by selecting the best gear for that road speed.
Ideally inside the vehicle owners manual
there would have been a
big bold colored graph
showing manifold vacuum on the vertical axis and
RPM on the horizontal axis.
Think this would have been too complicated for typical car owners?
Maybe.
But i bet you that if correct understanding of these lights and graph had been an extra credit item on the driver's license exam, many sixteen year olds ( 14 in Mississippi) would have ace'd it.
Today's teenagers do not seem to be highly motivated about driving, much less understanding automobile technology, but spend hundreds of hours honing video game knowledge.