I too doubt the missing glass will have a significant effect. It's possible that a little vacuum develops from wake suction inside the cap with the back glass gone. Integrated over the inside surface of the front of the cap plus bed areas might add up to some detectable drag force. Maybe. It's also possible that eddy currents developing behind the cap work a bit more favorably with the rear glass missing. Theory there being that eddies can tuck inside slightly and thereby draw the wake down slightly. Slightly. Maybe. Bottom line, I doubt it'll make enough difference to notice. I could be wrong. ABA testing before & after repair would of course settle the question. Meanwhile, enjoy your trip!
