As someone who lives in Canada, yes, winter fuel will hurt power output. Most winter fuels are "winter fuel" because their ethanol content has been bumped up close to the maximum allowable. This is to help prevent freezing in the gas line. Iceman likely hasn't had to deal with this living in Florida, I'm talking -5 F and colder temps being a concern. I deal all winter with people coming into our dealership complaining of poor mileage compared to their summer mileage. Between the slush on the roads and the fuel it's to be expected. That being said, you'd might still make more ultimate power on a dyno because you won't have to worry about heat soak. I tend to notice my car feels the fastest in the spring and fall when temps are cooler and I'm not using winter gas.
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