IDK man. Diesels have some pretty damn impressive numbers if done right. I wouldn't be shocked the least bit by 18 out of a truck like that. Then again, peddle down he probably only gets 3-4. Course that might just be the reefer talking.

 

I heard years ago that it has something to do with an air pocket that develops with the gate up. With the gate down there's no pocket so there's more resistance and with the cover there's no pocket/no added resistance but added weight...so it equals out. Soft cover for less weight or the mesh that helps keep the pocket with less weight seem to be the way to go.

 

If I were you, I would worry about the Philadelphia Parking Authority writing you a ticket for an expired meter then getting your tail gate stolen; they somehow come out of the woodwork like roaches once your meter is expired, almost like ninjas. Also, when down town, dont park on no stop areas I have heard peoples stories about the sign being covered by trees etc, they write you a ticket then tow your car. Just be careful. You should be fine other then that.

 

If your tailgate isn't down....It's out of mind. Keep it up. It holds the integrity of your box together as well.
I love seeing those guys with no tailgate and their box is flopping all around....lmao