Save money at the pump: Read your owner's manual
With prices so high, why waste your hard-earned money at the pump on something that doesn't do your car any good? If you buy gas containing a higher octane rating that your owner's manual recommends, you are doing just that. You may as well light a match to your hard won duckets.
Octane rating corresponds to the fuel's ability to ignite (from Wikipedia):
Higher octane ratings correlate to higher activation energies. Activation energy is the amount of energy necessary to start a chemical reaction. Since higher octane fuels have higher activation energies, it is less likely that a given compression will cause knocking.So that means that higher octane gas should give my car more pep, right? Wrong. Unless you're driving around with the pedal to the metal all of the time, you aren't reaping the benefits:
It might seem odd that fuels with higher octane ratings burn less easily, yet are popularly thought of as more powerful. The misunderstanding is caused by confusing the ability of the fuel to resist spontaneous ignition (pre-ignition = engine knock) as opposed to the ability of the fuel to burn (combustion). However, premium grades of petrol often contain more energy per litre due to the composition of the fuel as well as increased octane.
Even if you are a leadfoot, the benefits are modest at best if your car isn't designed for the higher octane gas. According to this study, a switch from 93 octane to 91 caused an unexceptional 4% difference in output when the engine is run under maximum load. The Federal Trade Commision puts it this way: "In fact, in most cases, using a higher octane gasoline than your owner's manual recommends offers absolutely no benefit."
Using high octane fuel for an engine makes a difference when the engine is producing its maximum power. This will occur when the intake manifold has no air restriction and is running at minimum vacuum. These conditions are present only when the accelerator is totally depressed. When this occurs, if a fuel with below recommended octane is used, then the engine will knock.
If you are to believe the advertisements, these "premium" gasolines also have more cleansing agents, which should have more benefit than lower grades of gas. That couldn't hurt, right? True, but "government regulations require detergents in all grades of gasoline" (The Straight Dope). Premium gas may have more, but 10 to 20 cents more per gallon for it? It's up to you.
Keep in mind that the converse is also true. Don't use gas with a lower octane than your owner's manual recommends. Most modern cars have "knock sensors that detect detonation and automatically retard the spark to compensate [...] eliminating knock but also giving you less power and poorer mileage." (again, from Cecil Adams at The Straight Dope).
Save your money, and read your owner's manual! You're already getting slammed at the pump.
Of course, it's never been a better time to invest in a hybrid.