6.0l vortec vs 8.1l with allison gas mileage

A 6.0 with the GM 6-speed seems to get about 20% better fuel economy unloaded than the 8.1L. But the difference is smaller when towing. There's so few 8.1L 6-speed allisons out there, I've never actually heard an owner's experience. The 5-speed Ally has about the same OD as the 4L85 so highway fuel economy will still be terrible.

For 7,000 pounds, the 6.0 should perform well enough, with the 6-speed at least. I was not a fan of the 6.0/4-speed combo. Too much spread between the gears.