My first towing truck was a 85C20....305...forget that. put in a 79'350 mildly modded, got 13 everyday driving and 8-9 pulling a 6K load, with a 700R4 in Drive not overdrive and 3.73's
2nd truck was a 96 Dually with a Vortec 454...12 mpg everyday driving, great creature comforts, 4.10's and 4L80E. 9mpg towing 8K load in overdrive @ 75mph.
3rd truck (not mine but I drove) a 03 8.1 Avalanche. 9mpg towing a 8K load, and handling absolutely sucked with it.
4th truck a 95 Burb. 350/4L60E 3.42's. again a great tow vehicle, but way underpowered for pulling 7500lbs. Worked its nuts off in Drive @ 65. 7mpg.
5th truck 99 2500 Silverado LT...my present truck. 6.0/4L80E 3.73's 11.5 everyday, 9mpg pulling 7500 lbs on mildly hilly interstates (midwest) I feel power is pretty good with this truck, but for the same mileage I was getting with the 454 dually, I would much rather have that drivetrain combo again.
Had a friend with a 2500 04 Dmax and we pulled 8-9000 lbs....effortless on hills and pulled down 14mpg with it doing it.