An octane rating is a measurement of how well fuels resist detonation. Modern engines use high degrees of compression. They therefore need fuel that does not detonate easily. Such fuel has a higher octane rating. The rating is done by comparing the fuel to the mixture of iso-octane and heptane that would detonate in the same way. Such a mixture of 90 percent iso-octane and 10 percent heptane has an octane rating of 90. Gas is available with lower octane ratings in the US than in Europe. The octane rating of gasoline tells you how much the fuel can be compressed before it spontaneously ignites. When gas ignites by compression rather than because of the spark from the spark plug, 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.