Trump will announce at a rally in Iowa Tuesday that his administration will allow the year-round sale of gasoline with up to 15 percent ethanol content, known as E15. Current rules cap ethanol content at 10 percent in the summer months.

The seasonal cap is one of the reasons that ethanol fuel use has lagged far behind expectations. It discourages investment in the specialized pumps required by the more corrosive higher-ethanol fuel. As a result, in many areas of the country E15 is not available at all.