Mark Chodas and I put together a project to test the viability of boat-tails as a way to reduce drag on semi-trucks. We built a 1/27th scale model for initial tests at the MIT Wright Brothers wind tunnel.
The tests showed a 40-50% reduction in total drag using the equivalent of a 3 meter tail. This translates to a 25% reduction in fuel consumption, or $35 billion per year in the US if every semi-truck was outfitted with a usable boat-tail.
Further tests tests were scrapped after it was discovered that there is a legal length limitation for drag-reducing airfoils of 5 feet :(