Welcome to EM!
I've got a 98 Civic EX with the grille blocked off entirely for winter, rear wheel skirts, and smooth font wheel covers. Those modifications in addition to a warm air intake have yielded me on average a ~16% increase in fuel economy (with pretty much all of the driving techniques outlined in the 100+ Hypermiling Tips section of this site). Doing any or all of these things is advisable.
You mentioned:
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I think it's somewhere in October where gas companies start adding fuel additives for the winter and then stop somewhere in May, like as you said my mileage would get better. Coincides quite nicely with the removal of the fuel additives so it must be true.
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Just to clarify, although the additives can be a detriment to your mileage, the main offender is the cold, which is why people install block heaters and block off all front grilles entirely during winter. Doing these things are also advisable, with objective supporting evidence on many threads you will find here. Again, welcome and happy modding!