You'll already be familiar with the Mustang's shape. While different from its predecessors, the car still has a distinct look, and Ford was keen to improve its aero attributes "without resorting to a characterless teardrop shape"--a little dig at some other fuel-sipping vehicles on sale today. Careful attention has been paid to the way air flows across the Mustang's surface panels, ensuring airflow stays attached--air bouncing about on the surface or causing turbulence increases drag, slowing the car down, causing wind noise and reducing economy.
Other details have changed too. The 2015 car keeps the Mustang's distinctive shark-nose grille, albeit smaller here than previous models. This means less air gets forced under the hood, where it can act like a parachute. Each of the three Mustang variants has a slightly different grille design, only allowing as much air through as each engine needs to cool and to breathe. On the turbocharged Ecoboost model, active grille shutters can adjust this to an even greater degree--opening wide for cooling, or closing to improve aero. Further preventing the parachuting phenomenon are the car's hood vents, which allow excess pressure to exit through the hood and over the car.
Wheels can be a major drag factor too, their spinning mass causing turbulence under the arches. Preventing this is a system Ford calls "aero curtains", where air drawn in through the bumper is forced through gaps in the arch, directing airflow over the surface of the wheels, rather than hitting it as they spin. It's a more aesthetic alternative to the best method of preventing drag from the wheels, full wheelarch skirts.
What works for economy also works for speed, in all these cases. Building a car that travels through the air with minimal resistance is great for economy, but Ford is confident its aero work on the Mustang has also resulted in a car that's quieter, more planted on the freeway, and better to drive at speed.