How much horsepower will lower octane fuel cost you?
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Seriously, it'd be indiscernible, any difference between 91 and 89 could easily be explained by a myriad of other environmental or gas quality factors.
If it isn't pinging, run the lowest octane she'll take, that's where max power is.
It's been well publicized that the difference between regular and premium gas is about 10 hp in the GT. Using 89 will put you somewhere in between. Certainly not enough to lose any sleep over. If I remember right, it's in the owners manual as well.
1- It is in the owners manual, read all about it.
2- On 87 octane the engine produces “only” 402 horsepower and 377 lb-ft of torque.
3- You bought a $40,000 car, don't cheap out on a few bucks a tank full...
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2011 Ford Mustang GT/CS
Performance White/Charcoal Black - Premium Package
Comfort Package - 6R80
How much horsepower will lower octane fuel cost you? 
-- 2011 Candy Red F150 Lariat 
