They do. See: [1] Between the back tires is a flap that helps with aerodynamics. I would say that 50%+ of the trucks I see on the road have these. [2] These a

They do. See: [1] Between the back tires is a flap that helps with aerodynamics. I would say that 50%+ of the trucks I see on the road have these. [2] These are much less common, but I'd guess 5% or so use these. [1] http://www.popularmechanics.com/cm/popularmechanics/images/pm/semi-trucks-05-0812-de.jpg http://www.popularmechanics.com/cm/popularmechanics/images/p... [2] http://www.blogcdn.com/green.autoblog.com/media/2009/11/part-450.jpg http://www.blogcdn.com/green.autoblog.com/media/2009/11/part...