b4000 gas milage issues in cold
 

I have a 2000 B4000 with a 4.0 that has new plugs and wires. I have had it one year as of this month and notice that my gas milage plummets when temps get in the 30's. It drops from a consistent 18-20 to a consistent 14 or 15mpg. Has anybody else had this issue? I have done the little stuff like fuel filter, air filter, plugs and wires and it only has 101,000 miles on it. Any advice would be great!

B3000 here. My '94 did the same thing year after year for 11 years. A steady 10km/lt in summer to a steady 8.5 in winter.
My '07 does it also but only about an 8% difference.
Don't know why.