Going on a Long trip in July (as you probably already know)

I have no AC working in the car and the sunroof is going to be open most of the way. I was wondering would a ventshade visor for my sunroof improve the aerodynamics over an open sunroof?

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RIP: 80 300SD
RIP: 79 450SEL
2002 E430 4matic (212,000km)
2002 ML500 'sport'

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