Here is something I found on the internet. Maybe this is why some of your stangs are pinging with higher grade. Read this
http://www.abcnews.go.com/sections/business/ dailynews/gas_scam010702.html
I thought you might keep it in mind the next time you go to the pump.
Brad
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1991 Mustang GT Conv.
Removed silencer, K&N air filter, Cobra manifold, 65mm TB,
Adjustable fuel pressure regulator, 180 thermo., Mac headers,
High flow H-pipe, Force II W/2 chamber Flowmaster,
Hurst Shifter, 3.55 gears, subframes