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Old 01-13-2021, 07:21 AM   #1 (permalink)
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Guessing at 'ballpark' changes in cD from changes in MPG .

I plan to eventually do some throttle stop testing on my car to estimate the cars' new cD, but for now, the geek in me would like to know if I can get a really rough guess at the change in cD based on changes in fuel consumption.

Here is a quote : To put Cd changes in perspective, Frasher put some numbers to a hypothetical sedan. Our imaginary car has a curb weight of 3,527 pounds, a Cd of 0.30, a frontal area of 23.7 square feet and 9 pounds of rolling resistance for every 1,000 pounds of weight.

According to Frasher, "If we put a gas-burning engine in this car, expect reasonable performance and drive it on a combined driving cycle, we can expect to get 23.8 mpg…. Add 10 percent to the drag coefficient, we'll now get 23.3 mpg…. Take 10 percent from the drag coefficient, we'll now get 24.3 mpg."

So I drive a 1993 Honda Civic hunchback.
The car has a approximate weight of '2,094' lbs. when new, with mine maybe being 100 lbs heavier with it's automatic transmission and aero mods I have added.

Dimensions: 160″ L x 67″ W x 51″ H for the frontal area

Rolling resistance ? ....no idea.

Stock, the factory quoted cD is .31

Highway MPG is 41 stock

So given these figures, what rough guess can we get at change in cD if there is an increase in MPG by

A ) 5 MPG
B ) 10 MPG

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Old 01-13-2021, 07:55 AM   #2 (permalink)
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It really does depend on what kind of driving you do. If it is all steady speed driving then it is a bit easier to calculate. If you are around town it is hard to calculate, if you are doing lots of cold starting it is hard, if the temperature varies so the tyre pressures changes,

For ballpark numbers at steady speed driving, the calculator in tools is very useful.

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Old 01-13-2021, 12:58 PM   #3 (permalink)
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In 2012, Frank Meinert, of GM Pickups, shared a ballpark metric with Edmunds.com.
Meinert suggested that, with 2012 pickups ( Cd 0.412 ), that any delta- Cd 0.01 drag reduction would translate to a 0.1-mpg improvement.
This is for EPA Composite mpg.
It works out @ delta-10% aero = delta-1.8% mpg.
EPA Highway would be higher.
Constant highway speed testing would be even higher.
The Rocketail Wing researchers were using Delta- 10% drag = 4.8% mpg, very close to the old 10-5 ballpark.

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Last edited by aerohead; 01-13-2021 at 12:58 PM.. Reason: typo
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