poor winter gas milage

What is your question?

I get 20-21 mpg on average with varying loads all throughout the non-winter months. Winter & cold temps lower this to 16-17. This is consistant. Why is there such a drop-off in fuel economy? I've been told it should be around 10% due to fuel additives, etc. I also have intermittant extended start-up cranking at various times with some loud muffler vibration at times too. The CT dealership I purchased from does not handle warrantee work any longer and never were very helpful so I'm looking for a local factory authorized service center if anyone cares to recommend one.