Extreme decrease in mileage

[OneBigD]

I've had my 2002 7.3 4x4 xtra cab (auto) since the end of last summer, and I have consistently gotten 18-20 mpg in it, no matter the driving (haven't towed my boat yet). Anyways, starting a month or two ago, my truck has been extremely difficult to start in sub 50 degree weather. It usually takes me a few minutes to get it started up. On top of that, my gas mileage has dropped considerably. I now get 14-15 mpg, and that is even when babying it. When it finally does start up, it usually shoots out black and blue for the first time, but then it starts up normally for any following start ups in the day.

What I have done:

-changed the oil to 5w-40 rotella synthetic
-drained the water/fuel separator
-replaced the fuel filter.

The only modifications to the truck are a 6" lift and 35" tires (both done prior to the problem).

Any ideas?

[mario a diangelo]

winter fuel, In the winter they add chemicals to the fuel so it does not gel or smoke excessively in the colder months. those chemicals burn much faster then diesel, hence the loss in mileage