If you are running too low of an octane for your motor, you will get
pinging. Pinging in its extreme form is also known as detonation. The
fuel/air mixture is igniting all at once and exploding instead of igniting
as a flame wave. The resultant "bang" is very hard on the pistons, head,
rods, and cranks. I've seen bent rods, cracked heads, pounded-out crank
bearings, cracked piston skirts, chainsawed crankcases, and thrown rods.
The only time higher octane is of any benefit is if the car pings on the
recommended grade, the timing is advanced, compression is higher
than stock, or the car has forced induction.
Newer cars generally wont knock or ping because they have
knock sensors that can compensate ignition timing for lower octane fuel.
You guys seem to know what you're talking about, so how about this - why do they sell the mid-range octane gas at all? Is it just a lure for people who think their Geo Metro will really have more oomph with a little higer octane?