Wow you reduced 10% of the weight O.o. I would be interested what other areas you reduced weight on.
I take it you also removed your alt since you don't use it now and saved weight there as well? Also how long have you used that battery like that for? That size of batter is probably so much cheaper to replace compaired to a full sized one
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I live in michigan, so winter 90% of the time means 20-40F at the end of fall and when winter actually hits it is normally around 10-20. This year is very warm in the 30s yet. Coldest times of the year hit below zero often. Having said that I have always had fairly large decline in MPG in the winter, but that was with warming up the car befor taking off. By large I would say ~22mpg vs 27 in the summer.