[aerohead]

Darin brought this up at the 'tire width vs drag ' thread and I thought I'd do a dedicated thread for broader access.
The reference I have which deals with this is:
"Tractive-Energy-Based Formulae for the Impact of Aerodynamics on Fuel Economy Over the EPA Driving Schedules"
by Gino Sovran,General Motors Research Laboratories
SAE Paper # 83034,1983
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The mathematics is very contextual,relying on a 'Representative' 1983 car of Cd 0.475,Frontal area 1.98 sq meters (21.3 sq-ft),Mass 1,279 kg (2,819 lb),Turbo-hydramatic transmission,radial tires;with 26 mpg Urban,41 mpg Highway,and 31 mpg Composite.
From Sovran,"There is no intent to develop a means for predicting actual on-road fuel economy level per se.Furthermore,the relationship between on-road and EPA dynamometer fuel economy is not addressed."
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Sovran develops an 'Influence Coefficient' which takes in Drag,BSFC,Driveline efficiency,Gear-matching,Cooling fan,Alternator,Power steering,Idling,Braking,Closed-throttle,Vehicle-to-vehicle repeatability,3-speed automatic transmission.
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Figure 3,Page 7 provides a table of mpg based on successive 10% drag reductions,for Urban,Highway,and Composite.
Here are some values for the Highway cycle with extrapolation out to a 65% drag reduction:
*0% = 41 mpg
*10% = 43.4 mpg
*20% = 46.5 mpg
*30% = 49.51 mpg
*40% = 52.11 mpg
*50% = 57.5 mpg
*60% = 63.25 mpg
*65% = 67.5 mpg
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The Federal Test Procedure for EPA "highway" mpg consists of a 10.2-mile 'trip' on a dynamometer,over 765-seconds,with:
* 1.62-mile 'Local' portion run at an ave. 40.7 mph
* 2.10-mile "Collector' portion run at an ave. 43.8 mph
* 3.96-mile 'Principle Arterial' portion run at an ave. 56.1 mph
*2.52-mile 'Minor Arterial' portion run at ave. 48.2 mph.
The entire 765-second test averages 48.2 mph for the composite test.
*Maximum velocity is 60 mph during the Principal Arterial portion.
*there is one complete stop during the test.
*Reference: 1972 Federal Test Procedure (FTP)
Passenger Car Fuel Economy During Non-Urban Driving
Thomas C.Austin,Karl H.Hellman,& C.Don Paulsell
Environmental Protection Agency
SAE Paper 740592,August,1974
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From Sovran's own words we understand that these relationships are limited to the dynamometer and do not present a useful tool to us as modders.