I have gone with many performance intakes over the years on a variety of vehicles. Lots has been said about using or doing away with an air box. What I have found is it makes absolutely no difference, the air box might help to prevent water from hitting your intake if you are driving in a lake but a separator like a K&N by no means is going to effect air temperature on a moving vehicle.
What is fact: A performance intake the cheapest $ per horsepower upgrade you can treat your PT cruiser to and open air gives you even more bang for the buck and more airflow.
Gas mileage.....a very high percentage of fuel savings comes from how you drive, if you like the increased horsepower and to "drive hard" you will likely not save fuel.
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Performancejeepchrysler.com for many performance intake options