If you hear a "loud" pinging sound or a rattle coming from engine it's a sign
your not using the right octane gas. A slight ping is not a big problem.

The problem is with timing pre-ignition, commonly called knock or ping
under load.  A small rattle up a big hill is acceptable, but if it
rattles every time you accelerate it will cause piston and crank bearing
problems.  Big problems......

The rattle or ping is the gas exploding under compression way before the
piston comes up to top dead center.  This kicks the piston back with a
hit you can hear in the bearing and cylinder wall tolerances.  The knock
or ping is from parts being violently slammed from one side of the hole
to the other against the normal motion.  This normally causes power
issues so it bogs down under load too.

How fast will damage occur?  Well you tell me which knock that one
piston with the weaker cast will decide to snap the skirt on.....

Then you come to gas mileage....  If your computer controlled car can
adjust to the lower octane, it changes the timing to do so, then the gas
mileage will start to suck.

I drive a non computer controlled engine and I get 100 miles distance
difference per 20 US gallon tank of gas when I use 91 octane 'high test'
vs 89 or 'regular' gas.

Even bone stock though, my Jeep owners manual says to use high test when
fully loaded or running hot and hard so I guess my 70 mph loaded to the
roof trips to get out camping and bush running count as that anyway.

Mike
86/00 CJ7 Laredo, 33x9.5 BFG Muds, 'glass nose to tail in '00
88 Cherokee 235 BFG AT's