Resetting Computer for different weather??

[eee]

Would there be much benefit in resetting the car's computer (mine is '95 obd 1 subaru)? I just got back to pdx oregon, from california, and the weather has gotten much much colder recently; real pacific nw winter weather. Anyways, I think the last time I reset my car's computer was in the summer. Would it make any sense to do this now-- due to the different temp's and the winter fuel?

2nd question: my milage has dropped about 10%, now at 25.5 mpg to 26.3 mpg these recent 2 tanks with 2/3 or more hwy. In the summer and much of the fall it's 27.5 to 29.5.