There is a misconception in the street-racing world that high octane fuel will make your car go faster or run better. This is simply not true. The octane rating essentially rates the fuel’s resistance to knocking.

Knocking or pinging is a sound that an engine makes when the air-fuel mixture in the combustion chambers ignites too early. Although slight knocking or pinging won’t damage your engine, loud knocking can cause damage and should not be allowed to continue. The solution is to use a high octane fuel, which is actually more difficult to ignite, despite the misconception that high octane fuel is more powerful. When the fuel is more difficult to burn, early ignition is eliminated, and with it the knocking or pinging.

The octane level required by an engine is determined by its compression ratio: higher-compression engines require higher-octane fuel. For example, a basic sedan generally only requires the standard octane fuel offered at gas stations, while a high-performance sports car or race car may require a high octane fuel. The owner’s manual lists information on the type of fuel you should use in your car.

Gas stations typically offer three different octane levels of fuel: regular, mid-grade, and premium. The regular grade generally has the octane level required by most cars, and mid-grade and premium are each a step up. Then, of course, there is the super high octane fuel available at race tracks, which is designed for the extremely high compression ratios that race car engines have. Pretty much all of the fuel grades commercially available have cleaning additives in them, so you needn’t choose a high octane fuel with the idea that it will clean your engine better.

In many European countries, and other countries such as Australia, the octane rating used is called the Research Octane Number (RON). These octane ratings reflect how the fuel acts in the lab. In the U. S., however, as well as a few other countries, the octane rating is called (R+M)/2. This number is derived from the average of the RON and the Motor Octane Number (MON). While the first rating is determined by the fuel’s performance in the lab, the MON is supposed to reflect how it performs "out in the real world." Therefore, an average of the two numbers should theoretically give the best indication of the fuel’s burning characteristics.

Basically, you’re wasting your money if you buy high octane fuel. You should start off with the octane level your car’s manual calls for; if you still hear knocking or pinging, you might try the next step up. If the problem persists, you’ll probably need to have a tune-up or some diagnostic work done. Don’t think that putting high octane fuel in your little Toyota will make it run like a race car – it just doesn’t work that way.

AND for those who whine about having to get 91 instead of 93 at high elevations... read up:

When I posted the comments above, I neglected to provide additional information about the importance of pressure in determining an engine's need for higher octane gasoline:

The presence of engine deposits not only increases the internal engine temperature, but also the engine's compression ratio. These deposits take space that would otherwise be occupied by an air-fuel mixture. This, in effect, increases this engine's compression ratio and it's need for higher octane gasoline.

Ambient air pressure is very important in determining the octane needed to prevent knocking. Normally aspirated gasoline engines (those without a turbo-charger) experience a drop of about one octane for each 1,000 feet increase in elevation above sea level. (Tests indicate that this elevation effect varies between 0.8 and 1.2 octane and averages about 1.0 octane). Engine makers normally design their engines to operate at or near sea level, and some (including Nissan) acknowledge that at higher elevations lower octane gasoline is acceptable. Almost all regular gasoline in the Rocky Mountain West (generally sold at 4,000 feet elevation or above) is produced at 85 octane (rather than 87 octane in the remainder of the USA). I do most of my driving around home at about 6,000 feet elevation or above. But at this elevation, 85 octane gasoline performs about like a 91 octane gasoline at sea level.

Moral of the story, 91 is FINE at high elevation, stop your whining... it's like the same as 93 at sea level.  Unless you're boosting all to high hell, in which you'd probably need race gas or weird shit regardless of where you live, go get 91 and stop your bitching.  There's a reason why octanes are lower at elevation.