It might seem silly to make hybrid versions of these big vehicles, but if you consider the amount of fuel ultimately saved, it actually makes a lot of sense. Even a modest improvement in the efficiency of a vehicle that uses a lot of fuel will save more gallons than a large improvement in an already-efficient small car.
There are performance challenges in creating a large hybrid SUV, though. If the hybrid version can't tow or haul just like the non-hybrid, consumers won't buy it. Instead they'll just go back to non-hybrid SUVs. So the hybrid SUVs have the same big V8 engines as their non-hybrid counterparts, ready to pull a trailer when needed.
When not needed, however, GM's "dual mode" hybrid system employs a variety of fuel-saving tricks when the vehicle is traveling at highway speeds.
Four of the eight cylinders will shut down when their power is not needed. (The hybrid uses a large engine - 6.0 liters - so that half will still provide adequate pull and the SUV can spend more time in 4-cylinder mode.)
The SUV's electric motor also connects to the wheels one way at low speeds and another at high speeds, allowing it to provide maximum assistance at any speed.
As with many other hybrid vehicles, the GM hybrid SUVs can travel under electric power alone for short distances at low speeds, and the gasoline engine shuts down altogether whenever the vehicle stops.