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Hyundai Elantra 2007- Horrible gas mileage. Went from 32 mgp city avg to 15 mpg city Please Help

I just replace the thermostat and flushed the radiator. The city mpg averages about 23-24, so it went up a little but still far from the 30s. When I turned on the car the other day after sitting unused for about 1.5 days, the rpms went 1500 while parked until the engined warmed up. Never experienced this in the cold weather. I’m guessing there is a pressure differential that is off since there is no engine light on. The catalytic converter may need to get replaced since it is an air filter of some sort.