Look at the side of your windshield. There are three letters, "as1" I believe. Tinting below that is illegal inmost states. I had my Black 01 Monte tinted 5% down to there. It was awesome, except when I was the first or second car at a light. I had to duck down to see the lights change. I had my 04 Monte and now my 03 GP done the same way, but 20%. Most shops will tint in a line from the bottom of your rearview mirror over to the "as1" mark.
Here's a link to tint laws, last updates July 07
http://www.iwfa.com/iwfa/Consumer_Info/auto_statelaws.html According to this link, it is pretty much illegal in Iowa to tint your front windows. Most glass has a visible light transmission of 85 and you can only go to 70.
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03 GT, STB, GM HD sway bars, 35% tint, GE893(37watt)fogs, Flowtech Terminators, 180 T-stat, ZZP DP & PCM, Ceramic PLOG & Crossover, 18" glasspack, 2007 17" Mustang Bullits 235/55/17 Pirelli PZero Nero, 12" R1 D\S rotors, gutted air box, GTP radiator