GM's two-mode hybrid system enables "full" hybrid operation with the capability to cruise on electric power up to 30 mph. The gas engine kicks in if more acceleration is needed, as well as at higher speeds; for maximum grunt, the electric motors and gas engine work together.

At the heart of the Sierra Hybrid is a 6.0-liter V-8 that teams with GM's Electrically Variable Transmission, which integrates two 60-kilowatt electric motors. The motors draw power from a 300-volt battery pack, which recharges itself off the friction generated when you hit the brakes. The engine makes 332 horsepower and enables a towing capacity of 6,100 pounds with rear-wheel drive and 5,900 pounds with four-wheel drive. That's not as much as the regular Sierra's 10,500-pound maximum towing capacity, but it's not too shabby for a hybrid.