E85 fuel is said to contain 85 percent ethanol and 15 percent gasoline, but the fuel mixture can vary anywhere from a low of 70 percent all the way up to 85 percent ethanol, depending on the place of refinement and the season of purchase. These E85 fuel testers will let you know exactly what you are pouring into your tank. Simply and accurately check the percentage of ethanol content in your E85 pump gas. Simply add water to the water line and then fill the rest of the way with your fuel in question. Shake it up and let it set for a few minutes and there will be a line of separation telling you what percentage your fuel really is - it's that easy! Comes with protective sleeve and instructions.