poor winter gas milage on 2008 Isuzu i290 Pickup

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.

Asked by for the 2008 Isuzu i290 Pickup
It sounds like your thermostat may be stuck open and so your engine is running too rich all the time because the engine never gets hot enough to run lean. Any Check Engine Lights on?