The 29 mpg EPA rating is actually an average of the Corvette Stingray's two standard driving modes, "Tour" and "Eco". In Tour mode, the Stingray rates 28 mpg highway, while in Eco, it scores 30 mpg highway. The main benefit of Eco mode for manual-transmission Stingrays is the activation of Active Fuel Management, cutting four of the eight cylinders under light loads. On automatic-equipped Corvettes, AFM is always active unless the driver engages manual mode via the steering wheel shift paddles. The automatic hasn't yet been EPA rated.
It's important to note that while the 911 and Stingray are right on top of each other on efficiency, the 911 generates 400 horsepower, while the Stingray outputs 455 horsepower--on essentially the same amount of fuel. Manufacturer performance estimates reflect the power differential: Porsche puts the manual-equipped 911 Carrera S's 0-60 mph time at 4.3 seconds; the Stingray's is 3.8 seconds (with the Z51 performance package). Both the 911 and the Stingray offer a seven-speed manual transmission.
The Corvette team certainly sees itself as a leader in this new aspect of performance-engineering competition. "The Corvette Stingray establishes the benchmark for modern performance cars by using technologies to deliver more performance and more miles per gallon," said Tadge Juechter, executive chief engineer for the Corvette. "We expect more and more performance cars will follow Corvette’s example."
Ultimately, gas mileage is still a relatively minor influence on the purchase of a world-class sports car--but when the cheaper, more powerful car is also more efficient, it certainly makes for an interesting consideration.