I had the sunrider top flipped up doing about 60 mph and the back window zipped up. A big gust of wind came and it became apparent why your supposed to open that back window. If that gust was any stronger, it would've picked the jeep up off the highway

 

Yup read your manual, do not exceed 40 mph with the Sunrider open!

 

hmm - I guess the open top fabric / frame could be damaged (ripped, torn, bent) at high speeds from excess + / - air pressure.