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I think 35% is the threshold for non-commercial vehicles with total blackout being permitted for commercial vehicles, food vans, ambulance etc. Most won't give you a receipt for the work if it's under 35% also I think, or they are very clear about the implications of it. I guess people like us don't attract the attention from the police like some young punk with a 20 year old WRX doing 85 in a 50 so we probably get away with it. Still, 20% is still too dark for sure. I wish I'd done 35% on my town-car.

Keep in mind the factory window tint your vehicle (probably) has. That's what tips my Landcruiser over the edge. The privacy glass in the rear windows. I have a darker tint on the front two to balance it but it's still not perfect. But yeah, it's a thing that needs some thinking about as it's costly to have to do it twice...Or get defected.