So many responses! TMI - almost!
Seriously, this is all great. I'll definitely do the math and work through all the ratios (engine, transmission, axle, tire size). These new many-speed transmissions may change things a lot!
But the irony? My question was to help me decide which axle ratio to order, only to have it pointed out to me that GM has taken away the option and only delivers the gas 6L in a 4.1 ratio! Decision made!
The hope was I'd be able to drive a single vehicle, getting 20 average mpg commuting while still dropping to the necessary 8.5 when towing. (I get 14 commuting now with my 2004 GMC 2500.) And maybe the right axle would help achive that. Doesn't look like it though.