Along with new manual and automatic transmissions  both with 6 forward speeds  the engines represent the single greatest technological leap for the Mustang since it entered production 46 years ago.

And while the new V-8 deserves its own Irving Berlin ballad, the tune today is in the key of V-6.

Let’s put that engine’s performance in perspective. It carries the same horsepower rating as the most muscular Mustang offered in 1998  the SVT Cobra with a 4.6-liter, dual-overhead cam, 32-valve V-8 that seemed exotic at the time. Yet the new Mustang manages 19 miles per gallon in the city and, when paired with the automatic transmission, 31 m.p.g. on the highway. That’s 5 m.p.g. better than the highway rating for the base Mustang notchback way back in 1984. That tired old horse weighed 900 pounds less than today’s larger pony, had a crummy 88-horsepower 4-cylinder under its hood and a gear-grinding 4-speed manual transmission.

“It’s all about interdependency,” said David Pericak, the 33-year-old chief engineer of the Mustang, explaining the talents of the new drivetrains. “About finding solutions at each opportunity that would be more efficient.”

Those optimized interdependencies let the car’s Duratec V-6 perform as if it were actually two separate engines: one optimized for fuel economy at low engine speeds and partial throttle, and one that roars when allowed to spin toward its red line of 7,000 r.p.m. at full throttle.

Start up the new V-6 and it growls a bit through its new dual exhaust outlets. Put the shifter in D, touch the throttle pedal lightly and the Mustang moves out, well, tenderly. There’s enough torque available from just off idle to motivate the car easily  even the heavier Mustang convertible. But if kept below 4,000 r.p.m., the engine responds modestly. It doesn’t feel as if it is making anything close to 305 horsepower.