All Sierras except the Denali come with a four-speed automatic transmission. What changes is the engine. Choices start with a 4.3-liter V6, standard on the WT Regular Cab/2WD Extended Cab models. One step up is a 4.8-liter V8, standard on WT 4WD Extended Cabs, SLE Regular Cabs and Extended Cabs, and WT/SLE Crew Cab models; that’s followed by a 5.3-liter FlexFuel V8, available on all; a 5.3-liter V8 standard on SLE and SLT models but available on all, and a 5.3-liter V8 with Active Fuel Management (AFM) on 4WD Crew Cab models. A 6.0-liter V8 with AFM is part of the SLE/SLT Extended and Crew Cab towing package. The Denali Sierra gets a 6.2-liter V8, mated to a six-speed automatic transmission.