Airtabs™ are particularly effective in cross wind conditions as they help minimize the flow of air across the gap to the "downwind" side of the trailer. If there is a measurable wind, inherently crosswinds will be a factor. Although the drag from a gap less than 18 inches is minimal, we recommend the application of Airtabs™ to combat crosswind induced turbulence and drag at the gap.
At the other end of the scale, Airtabs™ are producing excellent results with gaps as great as 60 inches.