Actually 91 octane USA is what they are built for. This octane rating translates to 95 octane in Europe which the cars are built for. If you put something lower than 91 thats when the knock sensors kick in.
When you run the car on regular gas the car compensates and retards timing. This reduces horsepower a bit and changes the air fuel mixture. So if you run regular, besides surprising the motor with something it was not designed for you actually get less fuel economy.
This is ESPECIALLY TRUE with the 430 and 500 motors. They really don't like regular fuel.
Obviously the car is designed that if you have to put it in for emergency reasons, it will function but making a habit of it to save 20cents a gallon is one of those "penny wise, pound foolish" things. You lose in the medium and long run.
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