There is no maximum distance apart, but prudence would dictate about 5-6". If we need, say 75 at 4" on center to have a 3% improvement in fuel economy effect at the back of a trailer, spacing them at 8" on center would probably give a 1.5% savings (half as many Airtabs™). We have no hard evidence to prove this, but it is a good rule of thumb.  Placing the Airtabs™ closer together than 4" causes some slight interference between the vortices produced, and obviously requires more Airtabs™.  Much as we’d like to sell more, closer spacing is counterproductive.