[aerohead]

The source of the CdA/mpg relationship is:
"Aerodynamics for Body Engineers"
by Kent B. Kelly and Harry J.Holcombe
General Motors Styling Staff,General Motors Corporation
SAE Paper No. 649A,January 1963,at the Automotive Engineering Congress
SAE Transactions,Volume 72,1964,page 571
"... a 10% reduction in aerodynamic drag makes a 5% reduction in fuel consumption possible at 55 mph,and a 6% reduction of fuel consumption possible at 70 mph."
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This relationship has absolutely nothing to do with the EPA.
It is for constant-speed top-gear cruising on a straight,level,dry,paved road with zero wind.
Typically,a small 'test' tank is in parallel with the main fuel tank,with a transfer valve operated by the test driver(s).As the car enters the test section of the 'track' the fuel valve is diverted over to the smaller test tank for the duration of the test period,then switched back to the main tank upon conclusion of the test.
By measuring the mass of fuel,before and after the runs,and knowing the constant velocity and distance,the mpg can be computed on a mass/volume basis.
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Today,with Scanguage and other 'instantaneous' mpg displays,anyone who can access a good stretch of highway ought to be able to construct a complete velocity/mpg baseline for future comparisons to deduce drag coefficients after mods.