My car idles at 1600 RPMs when I first start it.

My car idles at 1600 rpms when I first start it up in cold weather. Then slowly comes down to 1200. I let it run the other day for 45 minutes and it was still idling at 1200 rpms. Even if I drive a couple of miles when I stop it will still be at 1200. If I drive a distance the idling will eventually come down to about 7 to 8 rpm. So I am thinking no matter how long I leave it run it will stay at 1200. It is very cold here right now like below 0 but even at 20 above it will do the same. I am getting terrible gas mileage especially city driving. Sometimes I am down under 20 mpg. Also when it is cold it will stay at 1600 rpm for at least 6 to 7 minutes. Sometimes I get 32 to 33 on the highway but an average would be 27 to 28. The sticker says 31 to 39. I have never gotten close to 39 on the highway. Changes plugs and cleaned maf sensor. I bought the car with 103000 miles and expected to continuously get between 30 to 40 mpg but am not. Please advise on what to do to make this better. I also have poured in several containers of cleaner in the gas. I even filled up with a tank of premium and only got 28 mpg average. I was advised to do this from a mechanic to see what a difference it would make compared to the winter blend that we use here right now. Not much difference. Thanks and will be waiting for your reply. Corbett Severson