In MY 1992, the 2- and 4-wheel drive fleet distinction was eliminated, and fleets were required to meet a standard of 20.2 mpg. The standard progressively increased until 1996, when the Appropriations prohibition froze the requirement at 20.7 mpg. The freeze was lifted by Congress on December 18, 2001. On March 31, 2003, NHTSA issued new light truck standards, setting a standard of 21.0 mpg for MY 2005, 21.6 mpg for MY 2006, and 22.2 mpg for MY 2007
again just incase the edit was missed
So in 3 years the Cafe light truck standard increased 1.2 mpg which does not seem like much but when you are dealing with a vehicle in which the market buyer uses for occasional towing and hauling 22.2mpg is a ridiculous figure My old man rolls a 2005 silverado 1/2 ton 5.3 crew cab 5' box and gets an average of 16mpg mixed use.
So the V6's and 2 wheelers are helping the big three meet these standards but the loss of the separation of 2 and 4 wheel drive categories and the rate the standards are changing are causing big changes in the domestic light truck market.