historical origin of some rules of thumb
 

* Delta-10% Cd = delta- 4.2% mpg @ 70-mph. Re: SAE Paper 730790
* Delta- 10% Cd = delta - 2% EPA Combined mpg Re: SAE Paper 740969
* Delta- 10% Cd = delta- 4.28% mpg, EPA Combined, Re: SAE Paper 760187
* Delta- 10% Cd = delta- 4.236 % mpg @ a constant 55-mph for a fully-warmed test vehicle as per SAE J-1082 Road Test Procedure, Re: SAE
* Delta- 10% Cd = delta-5% mpg, @ a constant 55-mph, as otherwise tested to SAE J-1082 protocols, Re: General Motors Aerodynamics Laboratory, Warren, Michigan, USA.
* Delta 10% Cd = delta- 3.5% mpg, Re: Ford Motor Company, 1982
* Delta 10% Cd = delta- 8.82% mpg, Re: Robert Stempel, Manager, Chevrolet Motor Division, General Motors Corporation, Re: GM, Chevrolet Citation-IV concept car, 1984.
* ' If, in the given example, the drag coefficient Cd was reduced from 0.46 to 0.30 ( 34.782% ), a fuel consumption reduction for a petrol-engined vehicle of 14 percent would be returned ( delta-10% = delta 4.025% ). For a diesel-engined variant, a reduction of 17 percent would result ( delta-10% = delta- 4.887% ).' Wolf Heinrich Hucho, page 104, 2nd-Edition, December, 1986.
* A delta- Cd 0.01 = delta- 0.01 mpg, Re: GM Sierra / Silverado pickups, Frank Meinert, G.M. Pickups, to Edmunds.com, 2012.
* ' With an increase of 10 per cent in top speed, which results from approximately a 30 per cent reduction in aerodynamic drag,.............', Wolf Heinrich Hucho, page-92, 2nd-Edition, December, 1986.

You're undoing all the good work I've done.

I don't think you visit The Lounge, but that's where I went when another poster's ship ran aground on this very question.

The title was provocative. Zeroth order is less applicable than first order appriximation.

Delta- 10% Cd = delta - 2% EPA Combined mpg Re: SAE Paper 740969

Delta 10% Cd = delta- 8.82% mpg, Re: Robert Stempel, Manager, Chevrolet Motor Division, General Motors Corporation, Re: GM, Chevrolet Citation-IV concept car, 1984.

So what you are saying is that a 10% reduction in drag will give you anywhere between 2% and 8.82% MPG increase? To me that doesn't seem particularly useful.

"I got 2% increased MPG therefore I have 10% reduced drag" "I then did more mods and increased my MPG to 8% above baseline" therefore I can conclude that the further mods made no difference to aerodynamic drag because they both correspond to 10% drag reduction.

Correction:

Here's the article.

So what
 

Please re-read for specificity. All caveats and conditions are clearly indicated.

correction
 

Thanks! A typo on my part. Appreciate the eagle eye.

rule attribution
 

NASA used the 10%-5% relationship during their Edwards AFB research, and in their references, gave the National Science Foundation and the US Department of Transportation credit for it's origin.

Yes, in 1974 - nearly 50 years ago - and on very high drag trucks.

What has that go to do with current cars?

Nothing.

?
 

Whatever 'some cars' you mentioned at autospeed, in 2008, that ,members are considering for modifications.;)

10% / 5% @ ARC, 2021
 

@ ARC's website, they're beating the 10% 5% drum.
The Effect of Aerodynamic Drag on Fuel Economy | ARC