A good shop knows the local ordinances regarding tint percentage. You can go as high as the law allows, but consider that it means darker windows when driving at night, too.

Again, other than limo tint, you really won't make your windows opaque to curious eyes...

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2009 ML350 (106K) - Family vehicle
2001 CLK430 Cabriolet (80K) - Wife's car
2005 BMW 645CI (138K) - My daily driver
2016 Mustang (32K) - Daughter's car