Tom,

Thanks (in advanced) if you try out the 91 octane. I'm curious to see how much more oompf it will provide over the 89 octane. It may take a couple of tanks to make sure the HiHy electronics have adjusted to the higher octane level... If it also happens to provide a higher FE, that would be icing on the cake. You could make a general recommendation and perhaps get the general real-world FE numbers higher for the HiHy... The vehicle was designed to make use of that 91 octane.

The amount of FE improvement to justify the higher price in fuel is actually quite modest. If you churn the crank on the calculations, you can break-even if

FE1/FE2 = 2 - PG2/PG1

where FE2 and PG2 are the fuel economy and price per gallon for 91 octane and
FE1 and PG1 are the fuel economy and price per gallon for 89 octane.

We can simplify this even further and solve for the % increase in fuel economy required to break even:

%FE_increase = (FE2 - FE1)/FE1 = (PG2 - PG1)/(2*PG1 - PG2)

So, plugging in local numbers: PG1 = $3.21/gallon (89 octane), PG2 = $3.31/gallon (91 octane) this produces:

%FE_increase = 3.2%

So if our base FE is 25 mpg, then a 3.2% increase means that the FE must increase to 25.8 mpg to break even on the additional cost of 91 octane over 89 octane...