I don't know about non ethanal vs ethanol gas milage. But my 98's fuel mileage computer is very accurate, with the stock size tires. I think if you change the overall diameter of the tire, the readings might change. If you want to know exactly how different tire size affects your speedometer, go to the tire info section on Discount Tire's website. By knowing that, you can also figure out a change in fuel mileage.