The Short Story: When the Sierra hybrid is running on electric power, its transmission gears are infinitely variable, changing constantly but gradually as needed. You can take off and run on the truck's electric power up to about 30 mph, even if you are towing something behind you. Exceed that speed and the 6.0L V-8 gas engine kicks in, with transmission gears returning to a traditional shifting pattern. The gas engine economizes by running on 4, 6 or 8 cylinders, depending on how much power the truck requires to do the job.
The Sierra hybrid has a 26-gallon gas tank, and GM says to expect a 500 mile cruising range. That works out to about 19.2 miles per gallon, but it's unclear if that's a projection for combined highway/city driving.
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