The 2015 GMC Sierra 1500 is available as a 2-door, 3-seat regular side. Starting at $27,020, the 2015 GMC Sierra 1500 Base 4x2 Regular Cab 6.5' Box is powered by a 4.3L , 6 -cylinder engine mated to an automatic transmission that returns an EPA estimated 18-mpg in the city and 24-mpg on the highway. The top of the line 2015GMCSierra 1500Denali 4x4 Crew Cab 6.5' Box , priced at $ 53,465 , is powered by a 5.3L , 8 - cylinder engine mated to an automatic transmission that returns an EPA estimated 16-mpg in the city and 22-mpg on the highway.