Just to add a data point. I specifically sought out an 8.1L 2500 truck (mine is a 2003 Yukon XL). A 6.0 would have been fine for me but who can pass up an extra 100ft lbs of torque :smiley_drive:. The 8.1L is thirsty but it is a monster engine, especially going up grades and climbing steep hills. It has a bunch of torque right off idle which is great offroad. Nothing seems to really slow it down. It has drive by wire which is a little surgey as mentioned above but I will be able to work with it.
When I first bought it it had 3.73 gears and 32"/32" tires, average mpg was 10-12. I later added 35" tires, 4.56 gears, lifted it, added weight, added a solar panel on the roof and a walk-on roof rack and the mpg afterwards was nearly exactly the same. I also eliminated the stock front axle disconnect which means the front driveshaft is always spinning now. This past fall I did a ~4,700 mile trip from 15ft above sea level on the Gulf of Mexico to 10,000ft plus in Colorado/Utah/Nevada. My total trip mpg was 10.39, this was loaded down at 8,200lbs :Wow1:. Even at that weight the 8.1L didn't care and it felt about the same as driving empty. The engine drank about 3 qts of oil, which is not really that unusual for a big bore engine. Seemed to get about 11-12 on the highway and 8-9 offroad. Maybe I was into some winter blend fuels which would knock it down a little. It replaced a 95 V8 ZJ which could get better highway mpg but got as low as 6.3mpg offroad. The Yukon is not my daily driver so fuel consumption is not much of an issue for me.