apples to oranges but I have an Avalanche, which seeing you are GM tech and ASE Master (me to, although I let some certs expire, I'm done doing that) you would be familiar with. I figured I'd give you my data with a 5.3 liter.
Truck alone 62-73 mph for 90 miles, regular trip to cabin 15.8-17.2mpg average. If I keep it 55-65, which is the legal limit, half freeway half two lane I can get 19.8 ish, have had low 20-21mpg.
Truck with 17' lund 13.5-14.5 mpg, same with new 186GL. Usually also go 62-72 towing, only went the limit once or twice empty to see what it would do. Cover on the 17' adds maybe 1-2 mpg, haven't towed GL with cover.
All stock truck, LTZ with 20's. For what it is worth, never looked at ratio, never cared but I would guess 3.73?
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