The engine will run faster when it's cold. It's not unusual for the engine to run at 1500 to 2000 RPM when cold. If it's excessive you should have it looked at. 15MPG seems a bit low, but there is really nothing a dealer can do if everything checks okay. When a customer complains about poor gas mileage I check for proper tire inflation, make sure the brakes aren't binding or grabbing, and look at the engine data with a scanner to make sure that it's running within specs. If everything looks okay then there is nothing more that can be done.