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Hyundai accent mileage

Bought a new Hyundai accent in 2007. Have recorded all maintence & fuel added since (am a retired Army motor sergent). 36157 miles on the speedo. All scheduled maintence and replacement by dealer or Firestone. I hyper-mile and have averaged 41mpg for 5 years. On 18Nov12 my mileage dropped by 4-6 mpg. Replaced engine air filter and added “winter” air to the tires, didn’t improve mileage.
What could be the cause of sudden mileage decline?
How do catilitic converters degrade?

What is “winter” air? Do you mean you inflated the tires because it’s cold? To what PSI did you inflate them?

How are you measuring your mpg? Did the temperature drop significantly between your previous good mpg and your current lower mpg?

Maybe your gas station switched to a winter mix of fuel-that typically drops mpg’s a bit. It could also be a bad batch of gas. How many tanks of fuel have you had the lower mileage for?

Connect a scanner and check for stored fault codes.
Might have a problem with the ECT engine coolant temperature sensor or the MAP sensor.
Automatic or manual transmission?