I've calculated that by the time I 'graduate' ecomodder, I'll be saving at least $750 per month. Then again, I'm kind of going extreme in my 'modding' process
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The thing is that even when I was getting very good FE in my Geo (unlike now), I was actually driving more because I had the mentality of "ooh look! I can go further!". So my MPG increase really didn't do anything for my overall cost of driving.