oh yeah it is. in fact, with the windows down, it's even worse. windows down, and sunroof open, i'd guess you have added about 20% more wind resistance. i would be very surprised if the car would go over 140 like that, no matter what you have done to it. i know that in the cab, when it was stock, the top speed dropped from 150 with the top up, to under 140 with the top down, and that was a convertible, with no big wind catcher glass bubble in the back.
as i said, a more dangerous problem though would be rear downforce loss. you'll lose at least 50lbs of rear downforce. that's huge.
you want the air on top of the car, not in it or under it, or through it. close up every opening. tape up every body panel gap. seal up and smooth out every gasket. remove every little thing that can catch air. you would be amazed at how little things affect your speed. even wider tires, that have no body parts in front of them to direct the air around them, create more wind drag.
94 Midnight Metallic Blue Cab Porsche 968 w/deviating cashmere/black interior and WAY too many mods to list - thanks to eric for creating
www.968forums.com
"It isn't nearly as expensive to do it right as it is to do it wrong."
