Not only can cheap gas cause pinging, it will get considerably poorer fuel mileage than better stuff. The worst tank I ever got was along HW 40 in Amarillo.......I only got 22mpg, when I almost never get less than 30. No pinging, though.......I assume that tank had an unusually high percentage of ethanol.
And no, it was not E85......At least, the pump wasn't LABELED E85.