An aerodynamic device includes an inflatable member convertible between a deflated position and an inflated position at a speed certain. When in the inflated position, the inflatable member on the aerodynamic device extends between the cabin wall on a vehicle and a leading wall on a container supported by a trailer towed by the vehicle. The inflatable member fills a gap ordinarily between the cabin wall and trailer to encourage laminar flow as truck hauls trailer through an air stream. The encouragement of laminar flow reduces drag which results in increase fuel economy for the truck's engine, amongst other advantages.