10% = 5%
 

a search for Glen Scharpf, aerodynamics engineer for General Motors, landed on an article, 'AHEAD OF THE CURVE,' in the Chicago Tribune, February 12, 1998.
In the article, Glen attributed a 10% change in aerodynamic drag with a 5% improvement in fuel economy, at 55-mph.
This is what he used with respect to the Honda CRX-HF aero-modding data from CAR and DRIVER and Bonneville to come up with Cd 0.235, back in 1991.
It's not relevant to any EPA HWY test cycle, just constant-speed 55-mph cruising.
It may just be some internal metric specific to GM's Aerodynamics Laboratory.
So if you've seen me use it in the past, this is its origin.

Repeating the information doesn't improve its veracity.

For someone who claims to be an engineer, your maths seems very poor. I'll let you work it out for yourself, for if the Cd of a typical car has changed a great deal in the last 23 years, but rolling resistance has changed relatively little, how can that relationship still apply?

still apply
 

No where in my comment have I done anything other than provide a link to the past, with an explanation for the origin of a metric used by a professional aerodynamics engineer of the day.
I would really appreciate it if, you and Vman55 both, read for comprehension whenever I provide material.
Especially when you appear extra enthusiastic towards the unilateral judge, jury, and executioner.:)

By posting material, without qualification, you are deliberately misleading others.

Exactly as people in other forms of social media distribute and promote conspiracy theories (etc) by linking to them or distributing them (and then claim, "Oh it wasn't an endorsement, I just linked to it!".)

Despicable.

Please don't shoot the messenger, this from ARC (Automotive Research Center).


“The Effect of Aerodynamic Drag on Fuel Economy”
The Effect of Aerodynamic Drag on Fuel Economy | ARC

Very good. But of course, 'highway' can mean anything from 100 - 130km/h, and so that figure cannot be valid across those different speeds. I wonder what speed they're using for 'highway'?

And this was interesting:

"...as more and more vehicles are hybridized, then the regenerative braking reduces the effect of weight, and hence increases the proportion of the total losses coming from aerodynamics. For a high speed cycle with multiple acceleration and deceleration events this increase can be as much as 44%."

I hadn't thought of that.

deliberately
 

1) I did post material.
2) I provided the date of publication as a historical reference.
3) I provided a reference to a history that I share with Mr. Scharpf.
4) I attributed the relationship to Mr. Scharpf and a speculation as to its GM Aerodynamics Laboratory. ( GM has published other metrics like this, so there is a probability after the fact ).
5) The qualifications, caveats, conditions, are provided.
6) 'deliberately' implies that you possess my intent. That is unknowable, and the use of that kind of language can land you in a courtroom.
7) 'Mislead' can land you in a courtroom.
8) This topic has garnered interest. Providing a historical context is simply informational.
9) If the eye offends thee, pluck it out. Pluck it out.

You provided no qualifications, caveats or conditions on readers here using this 'rule' on their cars.

You just endlessly repeat the same stuff, attempting to justify whatever you've written in the past.

I think you are the number one source of misinformation on this subforum. You achieve that by a number of mechanisms.

1. Outright error (recent example - Reynolds Numbers)

2. Ignorance (recent example - what a TPS does in an engine management system)

3. Misleading posts (recent example - posting historic data without pointing out it may not be relevant to people modifying their cars today).

highway speed
 

For over twenty years, 55-MPH was the National Speed Limit in the USA. Any mention of 'highway' speed implied 55-mph. That began to end in 1995, as States were given permission to take back control of their own course.

English
 

Perhaps Australia offers remedial English and reading comprehension courses.
Perspicacity disorder syndrome. Context recognition.
This pedantry and semantics is already beyond growing old.