Most "premium fuel" cars built since the mid 1990s have a detonation (or knock) sensor(s) that will automatically adjust the timing to the fuel's octane rating. In some cases this may result in a detectable loss in performance, particulary low end torque. Most owner's don't notice a loss of performance with lower octane fuel, but it would be more likely on a boosted than a naturally aspirated engine.

If you want to experiment, here's how. When the tank is near empty, pump in about 5 gallons of regular unleaded. TURN OFF THE STEREO and listen for detonation. If none occurs, install another five gallons when it gets near empty again and continue the test. If you do not detect detonation, you can continue to use regular unleaded. If you do detect significant detonation, you can fill up the tank with premium. (No one who has ever done this experiement at my recommendation on a car less than ten years old has ever reported detonation.)

There is some variation among manufacturers, but most have multiple timing maps stored in the engine control electronics. The default map is set up for the recommended fuel, but if significant detonation is detected, the system will switch to a less aggressive timing map, and the knock sensor will continue to monitor for detonation and adjust timing as required.

As as example, the '05 Corvette LS2 engine has a 10.9:1 compression ratio. The fuel requirement is: "91 octane recommended but not required". Most manufactureres have gotten away from the "severe engine damage" warnings if you use less than the recommended octane because the engines are protected by knock sensors.

On pre-knock sensor engines, such as M102/103 you can usually use less than "premium" fuel, but you may experience some transient low rev detonation, especially in warm to hot weather or at high engine temperatures. It's possible to "drive around" the detonation by shifting at higher revs to avoid high engine load at lower revs or the timing map can be altered to something less aggressive by swapping EZL trim resistor. Detonation is typically more likely at low revs than high revs.

Duke