It sounds like the onboard computer wasn’t programmed for fuel with that much ethanol. It couldn’t tell it from bad gas and pessimistic parameters were used. You’d sacrifice something in energy content with an extra 5% ethanol, but that much makes no engineering sense.
The Silverado has a badge on the tailgate that advertises “flex fuel vehicle”. That is supposed to indicate a seamless transition between fuel types does it not?
No.. the 2012 sliverado is flex fuel. anything from E0 to E85 can be ran though it. He should not have seen such a loss in MPG but I wonder if it had anything to do with elevation or stop and go traffic or perhaps a pump that stopped a few gallons short of “full”