With a new V-6 version, the Mustang hits an all-time high fuel-economy rating of 31 mpg, according to the EPA's highway cycle.
By trim lines, the Mustang's gas-mileage ratings span a wide range. With the V-6 and a six-speed automatic, the official numbers are 19/31 mpg; with the six-speed manual, this version earns 19/29 mpg scores.
The V-8 engine doesn't cut into economy as much as you might expect. With the automatic, it's rated at 18/25 mpg; with the manual, it's 17/26 mpg.
The Shelby versions with the larger V-8 engine are significantly less economy-minded, as you'd expect. They're rated at 15/23 mpg.