The standard for "regular" fuel in the US these days is 87 and that is what you will see for the average engine designed to provide average performance. In many parts of the country, 87 is the minimum octane fuel you will see at the pump. However, in some higher elevations (like in Utah), you will see 85 octane gas sold, and for many average engines designed for 87 octane fuel, you can use the lower 85 octane fuel.

The explanation for why is a matter of air/fuel mixtures and air density at elevation....

Anyway, bottom line is that in some areas you can use 85 in place of 87. But that doesn't extend, in almost all cases, to using 85 or 87 in an engine designed to run on 91. Too big a jump. There will be performance losses.