You might have noticed that some gas stations occasionally offer specials on unleaded 88 octane gas—also known as E15—which contains 15 percent ethanol. Ethanol is an alcohol that’s derived from corn.

Editor's Note: APYs listed in this article are up-to-date as of the time of publication. They may fluctuate (up or down) as the Fed rate changes. CNBC Select will update as changes are made public.