I got into Hucho's book and located the material I mentioned.It wasn't what I thought.
It had to do with the EPA Composite Cycle:
*Lowering drag from Cd 0.46,to Cd 0.30 (a 34.78% drag reduction) would provide:
*a 14% fuel consumption reduction for a gasoline vehicle.
*a 17% '' for a Diesel vehicle.
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*It doesn't parse out a steady-speed,level road relationship.
*And it has more to do with the higher thermal efficiency of the Diesel in Urban driving than aerodynamic effects on the highway.
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*We might default to the earlier GM metric of a 5% mpg improvement for any 10% drag reduction at 55-mph.
*And a 6% mpg improvement for any 10% drag reduction at 70-mph.
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*The aerodynamic fraction of total load/power does increase with velocity,but as the drag comes lower and lower,there is a diminishing return.
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*What we're interested in,is the reduced power load,which directly affects mpg,based upon a constant BSFC for the engine which undergoes gear-matching to keep the new top speed rpm equal to the non-streamlined top speed rpm.Without gear-matching,we can lose up to 50% of the streamlining benefit,as the engine falls into an island of inefficient operation.
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*This makes it very difficult to predict things when we're not presented with a BSFC map for our vehicle's engine.
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*And this is aggravated by the fact that even if we get a top speed from say CAR and Driver,Road &Track,etc.,it won't actually represent the vehicle's actual potential,since top speeds are gear-limited perhaps in mid-gear,with 2-additional overdrive gears provided to slow the engine, at a constant top speed (at sometimes,much lower horsepower than rated power,but full torque),to reduce engine wear.
