Most new cars already have a slight tint that filters out most UV light. It does seem to me that adding an aftermarket tint helps me feel cooler while driving. It is like being in the shade vs out in the sun. My wife’s car does not have aftermarket tint, but it does have the “Solar ray” glass and it seems hotter when the sun is on my skin.
When the vehicles are parked in the sun, I don’t think the aftermarket tint reduces the inside temperature much if at all. Down here in the south, it gets hot inside a parked car no matter what you try to do. If there was a small difference in the inside temperature, I don’t think you would notice it.
All states have limits on the minimum allowable transmission of light for the glass. You have to be careful here though. A common acceptable limit is 35%, but if you have a 35% tint installed, you could be below the limit because of the UV tint already in the glass. You may need to get a 38% in order to be legal.
There are differences in the heat rejection of various tints as well. The transmission percentage is of visible light. One 38% tint may have significantly more heat rejection a
than another type of 38%. A good tint shop will not only list the percent of visible light transmission of the various tints they sell, but also the % of heat rejection. The tints with the higher heat rejection tend to cost more.
How the tint looks on your car will depend on the cars interior and exterior colors. My son and I have the same tint on our cars, done at the same shop. My son has a black car with a black interior and he gets pulled over frequently for a tint check. It always passes. I have a silver-blue exterior with a light grey interior. I never get checked.
If you see our cars side by side without anyone in them, you would swear that his has limo tint and mine is not tinted at all.
Something else to consider that I find extremely comfortable is a 5% tint strip across the top of the windshield. It can go down 6" or down to the AS1 line on the windshield. Before I retired, my commute to work have me driving into the rising sun in the morning and into the setting sun at night. That tint strip was a godsend and made my drive much safer.