[Myrddin]

It's a function of what the manufacturer recommends. If you choose an octane grade that is lower than the manufacturers recommendation, you may end up with pre-ignition aka pinging. This is caused by a more volatile combustion mixture igniting due to compression...before the spark plug does the job. It can destroy your engine. Higher compression engines require higher octane.

I live at a higher altitude (4600 ft). Our gas stations offer 85 octane. In San Diego (sea level), the lowest octane offered is 87. Buying a lower octane than required can result in engine damage. Buying a higher grade than required is a waste of money.

[Jeff Chandler]

. Buying a lower octane than required can result in engine damage. Buying a higher grade than required is a waste of money.

Agree. To which I add: buy the lowest octane your car will run well on. With modern electronic fuel injection and ignition, your car will adjust for the difference.

[oceanperch]

Thank You sooo much. I really did not have a clue.

I travel over the coast mountain range to get to Portland on a regular basis. Steep up and down to get over.

[oceanperch]

Thanks for starting the discussion.

[Publius6961]

Buying a higher grade than required is a waste of money.

Not quite accurate. it goes beyond wasting money.
I happen to be in the market for a new car and am looking at a hybrid (Toyota Prius) for commuting. I have been visiting as many web sites as possible to familiarize myself with the differences and new service requirements and technical nuances that go with the new technology.

Among the problems I found discussed is a strange one where the owner could not achieve the mileage the car is capable of, and it turned out to be that the octane of the fuel used was too high.

The difference in volatility was upsetting the programmed timing and totally destroying the efficiency designed into the engine.