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For the time being -and who knows for how long- Chevrolet and GMC will continue to offer a Duramax 6.6-liter V-8 diesel in their heavy duty pickup trucks. Recently, GM launched 2-Mode hybrid versions of the Silverado and Sierra that received an EPA estimate of 21mpg in the city and 22mpg on the highway (2WD versions). With the new 4.5-liter V8 diesel, it was estimated that the trucks would have achieved a fuel economy in the high 20s mpg.