You should always use the grade of fuel the vehicle engine was designed for. This should be quite clear in the owners manual and might also have a sticker at the inlet to the gas tank.
Using 87 octane in an engine that requires 91 octane will result in very poor performance, and your miles per gallon rate will drop significantly due to that poor performance, so there is no "savings" whatsoever by doing that. Sure, you will pay less for the fill-up, but you will fill-up more often than before ... It's a "catching the pennies while the dollars fly by" issue. The true difference at the fuel pump will be about $2 at most (on 14 gallons) on a complete fill-up ... isn't worth the trouble, really.
Conversely, putting 91 octane in car designed to run on 87 octane doesn't boost the power at all ... it may clean out the engine fuel system a bit, but if one changes the fuel filters and keeps up with the normal engine maintenance routine, then there is no need to to that either.
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