[Christ]

I only made the suggestion because I think that so little air will be able to get in with the lower pressure flow around the rim that there won't be enough "wind energy" if you will, to actually pull on it. Also, over time, a gasket rubbing will damage the painted surfaces on the back of your car.

If you don't feel it's going to be safe, by all means, use a gasket. We don't need the EM logo clearly displayed when/if the thing flies off and hits someone's car! :P (Assuming you'll put one on/in/around it, of course.)