welcome,
the first thing I would purchase (if budget permits) is a scangauge. look them up
www.scangauge.com you can get them other places cheaper but at least check out the info
also a grill block this will help with aerodynamics and engine warm up.
maybe a WAI (warm air intake) this doesn't work for everyone. if your car does compensate for temp, then it should help.
also put your car in the garage so that you can track your mileage and share info with others.
research is the key and this is a good place to do it. some of the guys on here have nearly doubled their mileage with aero mods and driving techniques. I am not quite as good as others but I am getting there.
good luck to you
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