Manufacturing efficiency. GM has a family of inline engines that all use the
same tooling and share many internal parts (4,5,and 6). Consumer Reports got
the following fuel economy for the 5 cylinder automatic crew cab Colorado:

CU's overall mileage, mpg 16
CU's city/highway, mpg 11/23
CU's 150-mile trip, mpg 19

CR got the following for the V-6 Frontier:

CU's overall mileage, mpg 15
CU's city/highway, mpg 11/21
CU's 150-mile trip, mpg 18

CR got the following for the V-6 Tacoma:

CU's overall mileage, mpg 17
CU's city/highway, mpg 13/21
CU's 150-mile trip, mpg 20

They are all pretty close as far as fuel mileage. This is what you would
expect given they are all roughly the same size and weight. (the Colorado is
actually the second heaviest, the Tacoma the lightest). The Colorado's fuel
economy probably suffers from having a four speed automatic. The Frontier
and Tacoma both get 5 speed automatics with the V-6 engines. Although the
Tacoma got the best fuel mileage, CR grumbled that it had the lowest load
carrying capacity, a poor seating position, less interior room ,and a bad
ride compared to others in the class.

Ed