10% = 5%

[JulianEdgar]

Originally Posted by RedDevil

I feel Aerohead put it in the banner like that because it is such a dead-beat horse. The fact that roughly about half the fuel consumption at highway speeds is due to aerodynamic drag is something that often comes up in explanations of car aerodynamics to the general public.

Maybe, But I don't take my aero knowledge from what the general public believes.

Some more accurate thread titles would have been:

- 10%=5%?

- 10%=5% at 55 mph in 1998

- Where I got 10%=5%

etc.

Nobody on this site believes that the 10%-5% ratio is a precise and immutable truth for all cars in all conditions.

Sorry, but I have seen it used in that way here - as a rule of thumb to be followed.

[JulianEdgar]

Originally Posted by ME_Andy

It's not Aerohead's rule, it's a GM engineer's rule.

Personally, I've come around to believing you know a thing or two and I'm happy to have you post here. I just want you to take a chill pill beforehand.

It was a GM rule back in 1998 at 55 mph. It was then picked up and spread by Aerohead here as the title of the thread suggests: 10%=5%

It is one of the many rules of thumb that Aerohead has put up here that, I think, cause far more harm than good.

Rules of thumb are bad news in any car modification area - suspension, engine management - and aerodynamics.

[freebeard]

Yes I did. I couldn't then be bothered trying to work out the obscure message.

Here is it without the punctuation

"0% reduction in aero drag does not automatically give a 5% reduction in fuel consumed (as Aerohead has stated). That [ratio]'s not my opinion; that's a zeroth-order approximation."

Since you can't be bothered to look at the link before posting:

Zeroth-order approximation is the term scientists use for a first rough answer. Many simplifying assumptions are made, and when a number is needed, an order-of-magnitude answer (or zero significant figures) is often given. For example, you might say "the town has a few thousand residents", when it has 3,914 people in actuality. This is also sometimes referred to as an order-of-magnitude approximation. The zero of "zeroth-order" represents the fact that even the only number given, "a few", is itself loosely defined.

A zeroth-order approximation of a function (that is, mathematically determining a formula to fit multiple data points) will be constant, or a flat line with no slope: a polynomial of degree 0. For example,
{\displaystyle x=[0,1,2]\,}x=[0,1,2]\,
{\displaystyle y=[3,3,5]\,}y=[3,3,5]\,
{\displaystyle y\sim f(x)=3.67\,}y\sim f(x)=3.67\,
could be – if data point accuracy were reported – an approximate fit to the data, obtained by simply averaging the x-values and the y-values. However, data points represent results of measurements and they do differ from points in Euclidean geometry. Thus quoting an average value containing three significant digits in the output with just one significant digit in the input data could be recognized as an example of false precision. With the implied accuracy of the data points of ±0.5, the zeroth order approximation could at best yield the result for y of ~3.7±2.0 in the interval of x from -0.5 to 2.5, considering the standard deviation.

Is there any other way I can help your understanding?

[JulianEdgar]

Originally Posted by freebeard

Since you can't be bothered to look at the link before posting:

I said that I looked at the link but then couldn't be bothered trying to work out the obscure message.

Thanks for the explanation but I don't think it's germane to what is being discussed here.

I don't think any ratio of drag reduction: mileage improvement is particularly helpful, because, as I have already written:

Before you can even start to describe such a relationship accurately, you need to know:

1. The starting Cd (a car with a Cd of 0.35 will have a higher aerodynamic contribution to overall resistance than a car with a Cd of 0.25).

2. If at a constant speed, what that speed is (aero drag goes up at the square of the speed; rolling resistance does not), so if the term 'highway' is used, we need to know what highway speed is.

3. If it's in a drive cycle test, what is that drive cycle?

...and there are other points as well, eg environment in which the car is being driven, absence/presence of regen braking, etc.

As soon as anyone introduces a rule of thumb, especially without heavily emphasising caveats, people will in ignorance subsequently blindly adopt it. Just as Aerohead did in another thread with:

Maximum torque occurs @ approx. A/F = 13:1 ( 'best power mixture' )

A perfect example of an incorrect rule of thumb in engine management!

Rules of thumb are bad news in any car modification area - suspension, engine management - and aerodynamics.