non-Mopar running issue

Please move if in wrong place
daily driver runs well, gas mileage went UP to 25 around town BUT when coming to a stop and putting the clutch in, it runs at an elevated (1500-2000) rpm for a few seconds before returning to normal. It's a 07 Ranger with the 2.3 and 5-speed and almost 90K. It would make more sense if mileage went down but it's gone up by one to one and half MPG. Thoughts? Also time to take in for emissions and I don't know if this throws a code that fails the test. Thanks for any thoughts and suggestions! (I'm going to look at the air filter tomorrow).