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.)