I have the 6.0L/3.73 in SUV form. Fuel economy has improved from new to over 30k miles now. I started around 12mpg mixed the first winter, and now we average about 14mpg in mixed commuting/around town as the main family vehicle. First long trip non towing got 15mpg in the 8k mile range, and last long trip @ 20k miles was 17.7mpg. This with a Rockebox on top and a hitch carrier on back. Towing: first summer was 7-8mpg. This summer is 9-10mpg so far. 65-70mph typical tow speeds. Mine hold 5th gear until some hills when I let speed fall to about 60 before a downshift to 4th when speed climbs back up.
I really like this truck. I would order 4.10 in a heartbeat, but not available on the Burb. 6th gear, 2000rpm, at 74mph. Seems a little low. And yes, it would tow more weight/higher wind drag in 5th gear. Fuel economy will not be measurably different under the same conditions.
Keep in mind the tow ratings and GCWR. 3.73 is only about 9500 pounds. 4.10 jumps to 14k pounds, much more than the 10% gearing benefit would suggest.
