2019 GMC Sierra AT4 enters winter with the option of some extra underhood oomph. On Monday, GMC debuted its new Off-Road Performance Package available for the Sierra AT4. With the optional package, the most rugged of Sierras makes 435 horsepower and 469 pound-feet of torque. That's up 15 hp and 9 lb-ft over the standard Sierra AT4 when equipped...
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