wow, I thought this thread was dead...
its all a matter of gearing and efficiency - here's one example off the top of my head
2007 Chevrolet Silverado 1500
LT2 4dr Crew Cab 5.8 ft. SB
Base Engine Size: 5.3 liters
http://www.edmunds.com/used/2007/chevrolet/silverado1500/100810025/specs.html
Fuel Tank Capacity: 26 gal.
EPA Mileage Estimates: (City/Highway)
Automatic: : 16 mpg / 22 mpg
2007 Chevrolet Silverado 1500
LT1 4dr Crew Cab 5.8 ft. SB
Base Engine Size: 4.8 liters
http://www.edmunds.com/used/2007/chevrolet/silverado1500/100810023/specs.html
Fuel Tank Capacity: 26 gal.
EPA Mileage Estimates: (City/Highway)
Automatic: : 16 mpg / 20 mpg
When you throw in the gearing variable of course things are going to change. I am assuming the 4.8L has either a tranny with one less gear, a different final drive ratio, different gear choices, etc... causing the lower EPA mileage results.