At 10:48 AM 3/11/2011, Dave Mcneely wrote...
>in general, only high compression engines benefit from higher
>anti-knock indexes than 87.  ... using a higher index fuel than
>specified is of no benefit.  It is not helping the engine any, just
>helping the fuel vendor's profit margin at the expense of one's pocket
>book.

I'm not so sure they make any more profit. Higher octane fuel costs
more to refine, and has lower yield. At the end of Nov '98, the
national average price for regular was $1.105, and premium was $1.303,
so premium cost ~18% more. The most recent prices were $3.52 and
$3.758, less than 7% more. The raw price difference only changed from
$0.198 to $0.206, less than inflation (~34%). Premium is a better deal
than ever, if you need it.

Sources:
http://www.eia.doe.gov/oil_gas/petroleum/data_publications/wrgp/mogas_history.html
http://www.inflationdata.com/inflation/Inflation_Calculators/Inflation_Calculator.asp