I have the hatchback opened, but sealed with a loose piece of plastic.... loose enough so that it snaps outward when the pressure is higher in the cab and snaps inward when the pressure from the rear is higher.
I noticed the following, regardless of speed:
Turn the vent fan on high, the plastic gently pushes outward, indicating a slightly higher cabin pressure than in the rear of the car (again at any speed).
Open a window ever so slightly and the plastic snaps forward into the cab, indicating a higher pressure outside the back.
Nothing open, the plastic is loose, lufting, indicating zero pressure differential between the inside and outside (center of the hatchback). Even at 60+ mph.
This latter result surprises me. I would expect some amount of vacuum near the outside back at speed, with respect to the inside of the car, but it does not seem to be the case.
Yeah,m my other cars have had good fan off air flow, but the VX is pretty bad in this respect (I think because the space between the hood and the cowling is extra tight and the air inlet is fully underneath the hood.
I think the explanation is relative "lack" of airflow... in that the inside pressure simply equalibrates with the outside rear, in absence of any strong pressure or vacuum introduction. The window open and the fan on high does the latter. It makes sense but indeed seem counterintuitive. Looking at some of the CFD simulations out there shows that the area immediately behind the hatch often has neutral air pressure with respect to the overall outside pressure. A foot or two back shows significant changes though.