What exactly is the Difference Betweeeen 87, 89, & 91??? AND WHY?

Unless the owner's manual specifically says you should use premium (91) or mid-grade (89) go with 87 octane. All gasoline sold in the U.S. now has detergent additives and a certain amount of ethanol, so anyone who tells you your engine will run better on premium doesn't know what they're talking about. The other problem is engines will develop a "memory" over time if you use higher-grade gasoline, which means if you use premium or mid-grade for a couple of months and then switch to regular 87 octane unleaded your vehicle will run rough and will get worse gas mileage because your engine has "become used to" the higher octane grade gasoline -- and it could run that way permanently. If your vehicle starts running rough on 87 octane regular unleaded, just put a bottle of Gumout fuel injector cleaner in it when you fill up -- it works GREAT!