If you filled the tank half with premium (93) and half with regular (87) you now have an octane of (87+93) / 2 = 90. If you had used the plus hose you would've paid the same price but only got 89 octane. So you got a higher octane for the same price. If you want 89 octane, use two parts regular and one part premium, not one to one. That will cost 6.7 cents more per gallon than regular instead of 10 cents.

 

short answer... dont go lower octane, more octane is a waste of money. Period.