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Old 01-20-2021, 04:08 PM   #1 (permalink)
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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.

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Last edited by aerohead; 01-20-2021 at 04:32 PM.. Reason: add data
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Old 01-20-2021, 05:06 PM   #2 (permalink)
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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.
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Old 01-21-2021, 06:25 AM   #3 (permalink)
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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.
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Old 01-21-2021, 09:54 AM   #4 (permalink)
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Quote:
Originally Posted by aerohead View Post
* A delta- Cd 0.01 = delta- 0.01 mpg, Re: GM Sierra / Silverado pickups, Frank Meinert, G.M. Pickups, to Edmunds.com, 2012.
Correction:

Quote:
GM's Meinert estimates that drag coefficients have improved about 30 percent in the past 32 years. A drag improvement of 0.01 equals a fuel economy increase of about 0.2 mpg in a typical car and about 0.1 mpg in a typical truck.
Here's the article.
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Old 01-22-2021, 12:31 PM   #5 (permalink)
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So what

Quote:
Originally Posted by AeroMcAeroFace View Post
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.
Please re-read for specificity. All caveats and conditions are clearly indicated.
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Old 01-22-2021, 12:41 PM   #6 (permalink)
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correction

Quote:
Originally Posted by Vman455 View Post
Thanks! A typo on my part. Appreciate the eagle eye.
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Old 03-26-2021, 05:01 PM   #7 (permalink)
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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.
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Old 03-27-2021, 03:42 AM   #8 (permalink)
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Quote:
Originally Posted by aerohead View Post
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.
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Old 03-31-2021, 10:17 AM   #9 (permalink)
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?

Quote:
Originally Posted by JulianEdgar View Post
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.
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Last edited by aerohead; 03-31-2021 at 10:18 AM.. Reason: date
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Old 03-31-2021, 01:12 PM   #10 (permalink)
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10% / 5% @ ARC, 2021

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

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