I believe mine is around 40% it was already on the car when i bought it. I personally don't like the real dark look. I also think that tinting the back darker than the fronts looks horrible. If you are gonna tint it use the same on the back and the sides. My friend just got 30% on his lexus is300 and it looks great I don;t think I would go any darker than that. It is hard to see in, but easy to see out without problems.
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