[RedDevil]

I have an in/out digital thermometer to measure air intake temp, but by location it also measures the temperature on top of my dash.
It can run off the scale (over 70C) with the windows closed.

When I leave the windows on the tiniest of crack it never runs so high. On the downwind side you can feel the heat escape through the crack.

I have a foam backed silver sunshade that I put over the windscreen when it is sunny. That's even more effective than (just) cracking the window.
I really wonder why so few people use them.
In the winter it keeps the window clean from snow and ice, and maybe just 1 degree warmer than without it; just enough to make it noticeably fog up less on start.