There should not be one.
The New York Times reports that when the price of gas drops people, in addition to buying more gas, also buy more premium gas.
Please, please do not do this.
It’s plain dumb.
A car engine needs a minimum octane rating to run smoothly. If the octane of the gas is too low for the level of compression in the engine, it may pre-detonate, ie, knock. The odds are high that your car was designed to run on 87. Anything higher will do nothing but empty your wallet.
That’s it. Adding premium is not going to make your car run any better, now or ever. (Unless it was designed for premium, in which case you own a sports car and your judgment is probably pretty bad to start.)
You may not own a sports car. You may just own a Volvo. Besides, isn’t this the miracle of the market at work? Are there greater externalities with premium gas or something? My assumption is that this rip-off operates mainly on the well-to-do. (PS–in Nevada, or maybe Wyoming there is 85 octane gasoline, which made my car a bit dyspeptic. Guess it’s because of the altitude?)
Altitude is the major factor in your reduced performance. Unless your engine has some kind of boost (turbo, super) then the air pressure going into the engine is the ambient pressure, and a given volume has fewer oxygen atoms than the same volume at sea level. By stoichiometry, you can only burn so many atoms of gas with that many atoms of oxygen. (no matter how much gas you squirt in) That’s why engines perform worse at altitude.
Knocking is driven by cylinder pressure, which is in part a function of air pressure, too, so they can use 85 octane at high elevations without knocking, and save a few cents per gallon. Note that the amount of energy available in gas does not depend on the octane rating. It’s just it’s tendency to self-ignite under pressure that changes. If anything, the problem with 85 might be when you drove back down from altitude with the 85 still in your tank.
I did elide a certain small truth which is that modern cars have knock sensors. If the engine detects knocking, it will retard the spark resulting in less performance, but safe operation of the engine. But this is only going to be a factor in engines designed for a higher octane.
Note also that E85 (which is 85% ethanol) actually has an high octane rating, above 100. But ethanol has less energy per volume than gasoline, so performance is reduced on that particular fuel. So if your car can take it, it’s worth experimenting with — if you think ethanol is a useful biofuel — a discussion for another time.