[aerohead]

Truck's been dead since 4-weeks ago today(we think it's ignition somewhere) and I've had a LOT of time to spend with the calculator doing forensic engineering with the 2012,and 2011 trip results.
I constructed a Road Load Horsepower Table based upon Hucho's Fig.3.11,'Influence of aerodynamic drag on top speed.'
I represented the T-100 at it's original Cd 0.44 and 28.925 sq-ft frontal area(estimated) and at 10% drag reduction increments.
Estimating a driveline mechanical efficiency of 92% you can just pick off a power requirement at any velocity or power setting.
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I took the closest mpg data sets between the two years,between the Salt Flats and Carson City,Nevada and back to best eliminate grade effects.(the boat-tailed T-100 and T-100 pulling the Viking were within 1.31% of each other on this round trip portion).
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I took Hucho's low-ball radial tire R-R coefficient and calculated the horsepower requirement to overcome Viking's tire drag,then added 8% to degrade it to a bias-ply tire performance.
Using a BSFC of 0.408 lbs/bhp I isolated the aerodynamic drag portion from the trip mileage of 31.330 mpg.
With the R-R out of the picture Viking would have indicated 32.991 mpg as just a boat tail.
Compared to an estimated 23.8 mpg @ 65 mph,the mpg indicates a 68.355% drag reduction [at 5.65% mpg per 10% drag reduction (interpolated from GM Research Labs)]the drag reduction would yield a drag coefficient of Cd 0.1392.
Due to the density altitude at the elevations I was driving at we have to discount the Cd by 9%,yielding Cd 0.151.
I'm going to include the elevations between the Salt Flats and Carson City so you can appreciate the implications of the trailers additional weight.
This simple 'look' at the data suggests that the trailer is performing as Hucho would predict.A wind tunnel would best measure the actual values,but for low-buck eco-modders it at least demonstrates a trend.