gas mileage!!!

[Sturdevant]

I'm not 100% sure of anything, so I use premium anyway.

[bados]

If you got a GT, why use regular? The performance difference is huge. Just use premium and have fun. I put on 33,000 miles a year and use premium. But I buy the premium at Costo where the difference in price can be 10-15 cents a gallon on the average. Cosco's gas is Amoco, so you can't go wrong.

[funksoulkitty]

The fuel and ignition maps that an ECU uses are based on what the anticipated octane level will be for a given car. For a factory ECU the amount of advance you have for a given cell on the ignition map (a cell being RPM vs load) will be what the car maker thinks is safe (ie, no detonation) while still giving you increased performance (the more ignition advance the greater the performance up to a point).

If you have a car with forced induction and you create a map intended for 91 octane, the amount of advance for a given cell will be greater than if you were creating a map for 87 octane. So, if you choose to use 87 or 89 octane when the requirement is 91, your car will still be following the map designed for 91 octane -- only now the poor little knock sensor and ECU will have to work overtime constantly retarding the timing to avoid detonation.

Your ECU has no way of knowing what octane of fuel you are using. It can only respond by retarding the timing if detonation is present as detected by the knock sensor. In other words, if you are perceiving a difference in performance (ie, your ignition timing is being retarded), your engine is experiencing detonation. Detonation is never good.

And, as someone else pointed out, why on earth would you get a turbo charged car and not enjoy it to the fullest extent?