about 9% of all gasoline sold in the us in may 2009 was premium grade, according to the energy information administration. some car manufacturers "recommend" premium gasoline but have computer-controlled engines that adjust the timing to avoid knocking. so, most cars can burn regular grade gasoline but at a slightly reduced performance. the associated press said premium gas–which is a higher octane and costs several cents a gallon more than regular unleaded–should be used only if the manufacturer says it is “required”.