I posted this in a reply to premium vs regular, but it seems nobody is viewing that thread so I thought i'd give it it's own. Here is a basic clarification of where octane comes from and how it is rated, and exactly why it is that there is no advantage to using a higher octane gasoline. Firstly, keep in mind that a gasoline’s octane rating is simply a measurement of the fuel’s ability to resist engine knocking. It does not refer to a substance or to the quantity of energy or power in the fuel. More correctly, an octane rating is often called an “anti-knock index”. The octane rating of gasoline tells you how much the fuel can be compressed before it spontaneously ignites. When gas spontaneously ignites, it causes knocking in the engine. Knocking can damage an engine, so it is not something you want to have happening. Lower-octane gas (like "regular" 87-octane gasoline) can handle the least amount of compression before igniting. To understand octane, you have to have a basic understanding of how fuels are refined. It is essentially a long string of hydrocarbons; every different fuel has a different number of hydrocarbons in the string. For example, methane is just a single hydrocarbon, propane has three, butane has four, pentane has five, hexane has six, heptane has seven, and octane has eight hydrocarbons in a chain. One thing the octane number tells you is what pressure it has to have to spontaneously combust, or explode without an ignition source…just pressure. The combustion due to the pressure and due to the spark plug is timed to be at the exact same split second, which makes a single even explosion in the chamber. Heptane explodes at a lot lower pressure than octane, and therefore it is mixed with octane to make gasoline that we use in our cars. The (R+M)/2 method is used to derive the octane number, however 87 octane also refers to a product that is approximately 87% octane and 13% heptane, or a mix that will produce the same reaction. Higher octane needs higher compression to explode it. The (R+M)/2 method is how it’s generally derived. The percentages of octane and heptane just happen to be how it’s mixed. The formula is as follows; RON (R), or Research Octane Number, over MON (M), or Motor Octane Number, divided by two. The RON is measured under fairly easy test conditions, while the RON is measured at higher engine speed and temperatures. Tetraethyl lead is a product discovered back around the WWII era. This was added to octane and hexane mixtures to decrease price. However lead is fatal to all forms of carbon based life (everything on the planet earth). This is still used in some airplanes that use a piston stroke engine (jet engines typically are burning kerosene). Now that you understand where the octane rating comes from, here’s something on engine knocking. Up until the 1985 mustang, carburetors were used to let air into the chamber, and they were very easily knocked out of alignment. So if air wasn’t getting in enough, this would cause a knocking with lower octane gas. Now that we have gone to fuel injectors, that’s where there air comes from…and it’s something extremely accurate that doesn’t need to be adjusted. So there is no need for a higher octane gas. However, with the lower compression engine, you have no need to use higher octane gas. You will not get a better mileage, because it is still compressing at the same rate, and it is not actually burning all the fuel. That’s why you may notice some perfectly fine unused gasoline dripping out of your exhaust pipes...however most of it turns into a gas state due to the heat. Hopefully that cleared a lot up.