In each EPA fuel economy category (city, highway, and combined), the new Mustang picked up an extra mpg. It now gets 16 mpg in the city, 25 on the highway, and 19 combined. It's not an extraordinary improvement, but it's not terrible considering that the engine also makes 25 more horsepower and 20 more pound-feet of torque than the previous model.
It should also be noted that this fuel economy rating is only for the 10-speed automatic Mustang GT. It's possible that the manual transmission model will have a different rating. On the previous model, opting for the six-speed manual would also yield an extra mpg on the highway (25 vs. 24). It's possible the new Mustang's fuel economy gain is because of the 10-speed automatic, so we wouldn't really expect any change to the current manual Mustang's fuel economy.