H3 V-8 MPG

Hi all, I'm new to the site, thinking of moving to a V-8 H3 from my 02 2500HD 6.0 Chevy. My 02 6.0 gets 11/14.5 MPG weighs 6200 LBS. My concern is why do the V-8's only get 13/16 MPG, when the chevy 4WD silverado with the 5.3 is rated at 15/20 MPG? Is it because of the AWD? Does the H3 have the DOD engine and use the same trans as the 1500 pickup the 4L60E I believe? Why wouldn't GM use the same powertrain package with the 5.3 as the trucks for a little better mileage? Seems like the H3 has some steep gearing in them. Does anyone know how many revs at 70 MPH they turn? Sorry for all questions. Great site. Thanks Dave

Box vs somewhat of a streamlined truck
 
The square box shape does nothing to enhance your mileage.  Also, AWD will play some MPG tax too... so will larger wheels, etc.

My first tank of fuel put my mileage at 15.8mpg's in the Alpha.

Wow...  I'd love to be able to track MPG more accurately in the truck.  15.8 average actually sounds great.  I monitor my fuel economy on a spreadsheet and my average over 24000 miles is right at 16 mpg with the 5 cylinder. 
 
As for what affects fuel economy, I read somewhere that the LED tailights in Toyota hybrids are projected to save 0.1 to 0.2 mpg alone over conventional bulbs.  So, basically, every little thing impacts fuel economy - obviously, though, the bigs ones are still gearing, aerodynamics, tires, AWD. 

You have to look at more than engine size for MPG.  The final gear in the H3 Alpha is somewhere around 4:1, I would imagine most pickup trucks have between 3.50:1 to 3.75:1 ratios depending what options come with the truck such as trailer packages.  When your rear gear is that low you have plenty of grunt but you pay for it at the pump. 

Notice that the Silverado's HP and torque are higher than the H3's too. I'm guessing here but they probably had a harder time fitting the 5.3L V8 into the H3 engine compartment and there could be compromises on the intake and exhaust that don't allow the engine to breathe as freely or operate as efficiently. They also may have changed over to some less expensive internal components that yield less power/mpg vs. the Silverado. As was said earlier, the 4WD operates in 2WD most of the time, especially during MPG tests and the H3 is full time AWD/4WD.

From Carlist.com http://www.carlist.com/autoglossary/autoglossary_41.html
This is the ratio of the gearset (usually the differential) that is farthest from the engine. A ratio of 3-54:1 means the driveshaft turns 3.54 times for every one turn of the wheels. Generally speaking, the higher the number, the better the vehicle's initial acceleration and pulling power. The lower the number, the better the vehicle's fuel economy.