I have the same problem, on an average week I do 99% city driving, and with hypermiling techniques I get about 18.5ish mpg, and that's really trying! I had a partial grill block, it helped my mileage a bit, but with summer in socal my engine gets too hot for my liking.
What I have on now is a partial front belly pan, it was extremely easy to make and it noticeably helps! I get into the high 30s and hit 40mpg once in a while! Without it I had hit 40 only once.
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