They actually vary from state to state, and the laws change all the time. For example in my state of Moooossouri, up until 2002, you had to have a permit to do ANYTHING on the front windows, had to have a state inspection at the Highway Patrol using their light meter, and THEN you could only go to 35% on the front rolldowns. Anything goes behind the drivers head. For 6 months (actually a screwup by the way the law was actually written by our lovely govt.), Oct 2001-Feb 2002, 35% on EVERY window, no permit needed anymore. This made THOUSANDS of cars immediately illegal across the state. Of course the state took advantage of the situation handing out tickets left and right. In Feb 2002, they re-wrote the law to what it is today: 35% on the front rolldowns, anything goes behind the drivers head, no permit needed.
Find your state, tint your windows: http://www.tintdude.com/laws.html
Notes on this website:
1. The AS1 they refer to is the little mark around 4-8" down from the top of your front windshield, most cars have factory tinted glass (not film) from the factory down to there.
2. the NET/Film column is just that: if your windows already have tinting on them, the total between the factory tint and the aftermarket film cannot be darker than listed in that column.
3. the lower the number the darker the film.
4. the 18-20% film is about what a typical SUV and/or minivan has from the factory in the rear if it has been tinted, just for reference.
5. I just noticed his chart hasnt been updated in a bit so check with your Highway Patrol for current regulations.