In Hawaii its 35% all around (+ or - 6%, so down to 29% is legal) on cars. Vans, trucks and SUVs are exempt on windows to the rear of the driver. So the front 2 have to be 35% (also + or -6%) and the rest can be completely blacked out if so desired. This exemption however only applies if they have "external rear view mirrors." I would think that most if not all other states have similar requirements, with the difference being the tint %. Generally you can find this information in the Statutes for your state. Alternatively I'm sure your local DMV has a copy of the tint law, but take it with a grain of salt. They tend to leave out some info that may let you get darker tint (they left out the + or -6% on their sheet here).