A GM tech explained to me that flex fuel vehicles have a ethanol ratio sensor so it can adjust the engine parameters to compensate for the wide range of ethanol one can use for best performance and maintain proper emissions. Non flex fuel vehicles do not have this sensor for obvious reasons and the computer is preset for ethanol ratios around 10% - 12%. The other problem is fuel system components on non flex fuel vehicles are not designed to handle ethanol content above 15%. So long term use will eventually do some kind of damage. Different manufacturers set up their vehicles differently to handle ethanol. Always follow manufacturers recommendations and warnings.