[aerohead]

For the Vortec 5000 @ 40-mph:
1) with your tires, the rear axle is turning at 424.82-rpm
2) the output shaft of the trans. is at 1584.58-rpm
3) and engine is at 1,191.41-rpm
4) running at approx. 23.64% brake thermal efficiency ( BTE )
5) and an available power of approx. 59.57-bhp
6) with maximum BTE occurring @ 77-mph,
7) and optimum HWY economy @ approx. 55-mph.
--------------------------------------------------------------------------------------
8) Your engine oil appears to already be virtually optimized.
9) same for the DEXRON-III Mercon, with pour point, cold & hot viscosity better than DEXRON-II, and as 'good' as PENTOSIN ATF-9, in use today with 20,000-rpm electric motors and supercar ZF transmissions.
10) The rear axle diff lube might benefit a smidgeon, if 100% synthetic, if it maintains the 'GL5' capability.
-------------------------------------------------------------------------------------
11) If your 'use' of the truck could be maintained with 'ALL-SEASON' radial tires, their lower rolling-resistance would have a high probability of demonstrating higher mpg at your target speed. 'All-Seasons' have the lowest power absorption coefficients of all passenger car tire types manufactured.
--------------------------------------------------------------------------------------
12) Under the ground rules for aerodynamics:
A) drag force reductions at 40-mph, will be 36% less effective than at 55-mph.
B) power reduction associated with drag reduction at 40-mph, will be 61.5% less effective than at 55-mph.
-------------------------------------------------------------------------------------
13) Since 'engine friction' IS 'hydrodynamic' friction, just as with an aircraft wing:
A) automotive lubricant 'friction' is 'RULED' by kinematic viscosity.
B) kinematic viscosity is ruled by ambient temperature and pressure.
EXAMPLE:
* 5W-20 full-synthetic motor oil has a viscosity of 340 centiStoke ( cSt ) at 0-C
105 cSt @ 20-C
47.8 cSt @ 37.8-C
and 8.1 cSt @ 98.9-C.
* In Vermont, on a 35-F day, the cold-cranking viscosity of this oil would be almost 42-times as high as on a fully-warmed summer day, where:
- engine oil @ 214-F
-coolant @ 203-F
-transmission @ 188-F
-Differential @ 159-F
-------------------------------------------------------------------------------------
The USEPA considers that tires are 'COLD', at 70-F.