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”.