I have a 2017 Outback 2.5 .  When first starting at under about 40F the engine revs at about 2000 RPM at around 30 MPH and does so until it is heated up to the point that the cabin heated will work. Also, under 40F the gas mileage goes down by at least 10% or sometime more.  Any thoughts ???

Oooops  that's a 2014 Outback

On older Subaru's high idle RPM when cold is normal.  

Gas mileage always goes down in Winter, all that sitting letting the car warm up.

Idle will be high for the first few minutes after starting. When its cold out it will idle higher until the engine is warm. The main reason is emissions control. A cold engine needs more fuel to stay running. The sooner the engine reaches operating temp the sooner it can run on a lean fuel mixture which reduces emissions during warm-up.

 

This is partly why you see a fuel economy drop in winter. The engine uses more fuel during that warm-up period.