[aerohead]

I messed around on the calculator to get a rough feel for the Golf.
Loose Specifications:
*Cd 0.30
*Projected frontal area 20.7 sq-ft
*Curb weight 2,800 pounds
*Coefficient of rolling force 0.015
*47 mpg at a steady 70 mph
*1.4893 gallons/hour
*10.173 pounds diesel/hour
*Aerodynamic Road Load @ 70 mph 77.882 pounds/14.539 hp
*Friction and tire losses @ 70 mph 7.84 hp
*Total Road Load @ 70 mph 22.379 hp
*Required engine power @ 70 mph 24.325 bhp
*BSFC 0.4182 lbs/bhp-hr
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If I leave the car alone,but increase the velocity to 80 mph the engine power requirement rises to 33.328 bhp.
*Rolling resistance tractive power is up to 8.96 hp.
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To achieve 50 mpg @ 80 mph requires:
*1.6 gallons/hour
*10.9286 pounds/hour
*dividing by BSFC = 26.133 bhp
*= 24.042 hp
*minus 8.96 hp rolling resistance leaves 15.082 hp for aerodynamic drag.
*Solving for the Cd using the standard aerodynamic power equation yields Cd 0.208 necessary to produce 50 mpg @ 80 mph.
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*Cd 0.208 is 30.5% lower than Cd 0.30.
*A 'typical' belly pan would yield about a 12% drag reduction and 7.2% mpg improvement @ 70 mph.
*Even the 1982 Audi 100 III with Cd 0.30 yielded only Cd 0.23 with a full belly pan with diffuser.
*The Golf has a smoother underside than the 1982 Audi to begin with and the best underside would yield less of an improvement than what Buchheim et. al. achieved with the Audi after $1.5 million in wind tunnel time (1982-Dollars).