So I got this car used with $119,000 miles. Had it for 2 months I had to have the heater core replaced , gasket and the serpentine belt and water pump. Oil been changed. I wasn't getting very good gas mileage and I reset it on the entertainment controller. It finally went up to 35 on the highway and lower in the city but now it's really low. Almost 13 to 16 in town and 22 on the highway. I don't know it's cuz I reset it or if anybody has some information I'm missing..thank you

 

Keep driving it and see if the MPG remains the same or begins to go lower.
Thus far nothing is out of the ordinary and MPG has way too many variables that can affect the changes in MPG. You could go to a parts store that offers free ECU scanning just to see if there are any active DTC even if your dash check engine light is not on? But keep in mind most only look at active DTC. You can ask them to check the I/M monitors while doing the scan. But don't be surprised if they don't know how to do that.
If anything is found, you can post back the findings.