TEST-DRIVE: 2015 Mercedes Benz S550 Plug-In Hybrid. “The 114-hp electric motor is integrated inside the seven-speed automatic, and it is powerful enough to propel this S-class to 87 mph on electricity only. Using a light touch on the accelerator, it can go for 20 miles on a full charge before the direct-injected, twin-turbocharged 329-hp V-6 kicks in.”