[aerohead]

A) it would be helpful to know GM's motor oil recommendation for 'winter' operation.
B) does your transmission use DEXRON-II ATF ?
C) GM's rear axle/differential lube recommendation , SAE 90W, SAE 80W-90 ?
D) being a 4X4, are your tires different than P 235/75R15?
E) is there any chance that you could run 'All-Season' radials instead of LT-rated tires?
F) is the 'K' taller than the 'C'?
G) is the curb weight different than 4,313-pounds?
H) the C-10 that your body style replaced had a frontal area of 32.7-sq-ft, and the K-10 was a bit larger because of ground clearance and wider tires, at 34.3-sq-ft. My thought is that your K1500 is more like 34.3-sq-ft, than 28-sq-ft.
I) the normally-aspirated 5.0-liter's torque peaks @ 2,800 rpm, with 134- bhp available at the flywheel.
J) the engine's brake thermal efficiency ( BTE ) won't be any better than about 36%.
K) if your average 16-mpg is occurring at an average 40-mph, you're burning 2.5-gallons per hour, or, 279,590- Btus/ hour ( 109.8-horsepower worth of heat ) and at a BTE of 36%, it leaves you with 100,652.4-Btus-brake/ hour ( 39.53-horsepower of useful energy ), before engine accessories and powertrain losses eat into that.
L) what's left over will be your 'Road Load', for rolling-resistance and air drag.
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* A tachometer would tell you engine rpm @ 40-mph.
* Tire size will allow rear axle rpm to be calculated for 40-mph.
* This will inform us about 'gearing' issues, keeping the engine near it's 'sweet-spot'.
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According to the ground rules for road testing, you'll need to drive the truck at a continuous 55-mph, for at least 23-miles before you can actually begin testing. It takes this long before all critical components, lubricants, and tires reach whatever equilibrium temperature they're going to experience at the current ambient outdoor conditions. If you fail to fully-warm the truck, whatever numbers you get, could be off by 20%. A complete disaster!