theoretically I'm sure it's not best for aerodynamics to run like that, since you have the rear window up, it basically makes an air pocket inside the car. Whenever I had my del Sols, I almost always had the rear window down, rain or shine. I tried running my car different ways at the strip, once with all windows up and top on, once with side windows up and top off and back window down, once with top off all windows down. On a GSR powered del Sol, it didn't make any difference. My times were all pretty consistent, and I didn't notice any major differences with the top on/off, window up/down combinations, etc.