13a First you fill the test tube with water up to a certain line marked on the body of the tester. Then you pour in E85 until it reaches the max fill line. A few easy shakes to properly mix the two fluids and you get a percentage reading. Our fuel was over 85 percent but less than 90 percent. E85 stations advertise a minimum rating of 68 percent ethanol so be careful, as your tune-up will be effected by this percentage. The station we use has a consistent 85-90 percent reading.