[aerohead]

I ran back-of-the-envelope calculations for Don Sherman's data, presented for the above, non-hybrid stable mate of the Tahoe.
1) using the reported 20-mpg, and using 70-mph ( 115- km/h ), yielded 3.5-gallons per hour.
2) multiplied by 6.138-pounds/ gallon ( reformulated E10 Regular Unleaded gasoline ) = 21.483- pounds/ hour.
3) 44- horsepower Road Load / 85% driveline mechanical efficiency ( a value used in the past by CAR and DRIVER, and perhaps from the BOSCH Manual ) = 51.7647 brake- horsepower at the flywheel.
4) 21.483- pounds fuel / 51.7647-bhp = BSFC 0.415 pounds/ bhp-hour.
--------------------------------------------------------------------------------------
5) the 2008 Tahoe's reported projected frontal area is 88.413% of Gross frontal area.
6) using the Width and Height of the Denali, and the 88.413% relationship from the Tahoe yields an appox. Af 36.772 square-feet ( 3.416 sq-meters ).
7) plugging in Cd 0.36 yields CdA of approx. 13.237-sq-ft
8) using the drag power formula at 102.666-feet/second ( 70-mph ) yields a road load aerodynamic horsepower = 30.9948- hp.
9) subtracting 30.9948 from 44.0 = 13.00515 hp due to rolling resistance.
10) using the rolling-resistance power formula yields a rolling friction force coefficient of 0.010954543- pounds/ pound ( for the set of four (4) tires, @ an SAE test-weight of 6,360-pounds .
--------------------------------------------------------------------------------------
11) lowering the Cd to 0.10, and using the velocity-squared fraction, compared to Cd 0.36, yields an aerodynamic horsepower of 8.609666-hp.
12) adding in the same rolling-resistance horsepower ( the tadpole's all-up weight is equal to a single passenger ) yields a new total road-load horsepower of 21.6148-hp ( compared to 44.0-hp ) @ 70-mph ( 115-km/h )
13) 21.6148-hp / 85% mechanical efficiency = 25.42919 bhp at the flywheel.
14) 25.42919 bhp x BSFC 0.415 = 10.553- pounds/ hour.
15) 10.553 / 6.138-pounds/gallon = 1.719-gallons/ hour.
16) 70- miles per hour range / 1.719-gallons = 40.71-mpg @ 70-mph ( 115-km/h ), for the non-hybrid Yukon.
--------------------------------------------------------------------------------------
This is for 40,000-miles of driving, under four-seasons conditions, wind spectra, and topographical elevations changes.
--------------------------------------------------------------------------------------
So, shooting from the hip, it looks like the Tahoe ( with the hybrid advantage ) would have a shot at 40-mpg, even at 70- mph, @ Cd 0.10.
This assumes a constant BSFC, which requires matching the engine load to that of the original OEM 'Map' conditions, typically resolved with taller gearing.
--------------------------------------------------------------------------------------
While ' technically feasible', at 100-years on now, we can stand back and watch the arguments fly.