This is a question which is currently in debate between the Environmental Protection Agency (EPA) and the automotive industry. Currently the standard for ethanol blend gasoline is 10 percent. This is the percentage that vehicles currently in production today are being manufactured to run in their fuel line, and are being warranted as such. So if you ask an automotive manufacturer just how much ethanol a regular gasoline engine can handle, they will tell you 10 percent and no more.
Now, there are other interest groups that have tested these vehicles with much higher blends and have petitioned that these vehicles can handle as much as 20 percent ethanol without any problems developing. The problems that can arise with high blends of ethanol, are corrosion in the fuel systems that were not designed for use with ethanol biofuel. A vehicle needs to be manufactured with special components in order to run the E85 blend gasoline (85 percent ethanol). Auto manufactures do not want to be held responsible for fuel use beyond their manufactured standards.