I'm looking at GMC/Chevy 1500s, in the 04-05 range, with a 5.3L v8, 4-wheel drive, 3.73 axle ratio, and probably with the Z71 package.  I believe the base tow capacity is 7800# for these specs.  

I'm finding a few possibilities that have upgrades in engine, suspension, and axle ratio, and I have questions about the effect those would have on the towing capacity and gas mileage.  (This truck will mostly be our second commuter vehicle so gas mileage does matter.)

HD version (boy that's hard to find in a 1500 but I HAVE found two) - Does this affect the tow capacity or gas mileage?

4.01 axle ratio - By itself this makes the tow capacity 8700#.  How much impact does it have on the gas mileage?

6.0L engine - By itself this makes the tow capacity 8700#.  How much impact does it have on the gas mileage?

What about a 1500 with the 6.0L engine AND the 4.01 axle?  What is the tow capacity and gas mileage?

Thanks!  I've looked everywhere, even on the GMC website, and can't really find the answers I need.

You may want to ask here, you'll find more 1500 HD's in the RV world... http://www.rv.net/forum/index.cfm/fuseaction/listings/forum/41.cfm

or here, in the "gasser" section..  http://www.dieselplace.com/forum/forumdisplay.php?f=9

1500 HD is made 2001-2003 then they had a light duty 3/4 ton in 04 then went back to 1500HD it is only available in 3.73 gears with the 6.0. The 6.0 can be a gas hog depending on how you drive.

I have a 2002 2500HD Crew Cab short box with a 6.0 with 4:10's I get 14-16 on the hwy with out a trailer and 12-13 in town. With a bumper pull I get 10-12 and with a gooseneck on the highway I get 12-14. the truck is stock except my exhaust. I have a custom dual exhaust system and I have one size bigger tires on the truck. The trucks are a great work horse I have 110,000 on mine now the only thing I did not like was the GM coolant fluid so I switched it out to a green fluid.

In the 2006 and newer have ton trucks they have a vortec max that is the only regular halfton that has the 6.0 in it.