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Old 06-07-2021, 07:51 AM   #1 (permalink)
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Humor me, I have lowered my trucks Cd to .32?

I think the Cd on my truck is .32.

.32, down from .39 factory, with just a half tonneua cover and lowering.

Using the Aerodynamic and rolling resistance tool, I figure my Cd is .32

Here's the variables I used-

Vehicle weight: 1950.4 kg / 4300 lbs
Crr: .008
Cd: .32
A: 2.7 m2 / 29.5 ft2-Down from stock 29.75
Fuel energy density (Wh/US gal.): 33557
Engine efficiency: .26
Drivetrain efficiency: .97
Parasitic overhead (Watts): 0
rho: 1.225 kg/m3

Varables that I know to be true-
Truck weighs >4300 pounds with me in it
Frontal area stock is 29.75
advertised Cd stock is .39

I took a 762 mile round trip, using 10% ethanol, 87 octane gas traveling @ 70-73 mph ( measured by GPS) I got 27.14 mpg.

Does lowering the truck decrease frontal aera more than just .25 sq foot? I figured that by less tire tread being exposed, because it's now further in the wheeel well.

So is my Cd really .32 now or I'm I tripping.

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Old 06-07-2021, 01:17 PM   #2 (permalink)
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So is my Cd really .32 now or I'm I tripping.
Sounded likely right up until the last three words.
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Old 06-07-2021, 02:42 PM   #3 (permalink)
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Originally Posted by freebeard View Post
Sounded likely right up until the last three words.
so you think my Cd could be .32 now?
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Old 06-07-2021, 05:10 PM   #4 (permalink)
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"Likely"

I was commenting on the 'I' for 'am'. Sorry Excuse me?
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Old 06-07-2021, 05:13 PM   #5 (permalink)
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Quote:
Originally Posted by Vwbeamer View Post
I think the Cd on my truck is .32.

.32, down from .39 factory, with just a half tonneua cover and lowering.

Using the Aerodynamic and rolling resistance tool, I figure my Cd is .32

Here's the variables I used-

Vehicle weight: 1950.4 kg / 4300 lbs
Crr: .008
Cd: .32
A: 2.7 m2 / 29.5 ft2-Down from stock 29.75
Fuel energy density (Wh/US gal.): 33557
Engine efficiency: .26
Drivetrain efficiency: .97
Parasitic overhead (Watts): 0
rho: 1.225 kg/m3

Varables that I know to be true-
Truck weighs >4300 pounds with me in it
Frontal area stock is 29.75
advertised Cd stock is .39

I took a 762 mile round trip, using 10% ethanol, 87 octane gas traveling @ 70-73 mph ( measured by GPS) I got 27.14 mpg.

Does lowering the truck decrease frontal aera more than just .25 sq foot? I figured that by less tire tread being exposed, because it's now further in the wheeel well.

So is my Cd really .32 now or I'm I tripping.
My best guess is that (in your words) you're tripping.

I think you can basically ignore all the calculations (BS in = BS out) and just look at the magnitude of changes required on real, properly-measured cars to see that change in Cd.
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Old 06-07-2021, 11:19 PM   #6 (permalink)
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Quote:
Originally Posted by Vwbeamer View Post
so you think my Cd could be .32 now?
I'll humor you... your assessment seems to show you have reduced your drag. Calling it 0.32 instead of 0.39 is using the calculations for the same rhetorical purpose as saying in p,ain English that you reduced your drag signficantly. It's a useful thought device, like creating a visualization of data. What you shouldn't imagine is that these calculations can stand in for a wind tunnel, but I am certain you know that. It is fun to imagine how the drag reduction might look numerically. And there is a general truth value in it.

I say congrats on improvements!
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Old 06-08-2021, 06:40 AM   #7 (permalink)
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It would be hard for me to believe my simple mods reduced drag that much. Thanks for the comments.

One thing for sure, lowering the truck did drastically improve the gas miliage.

Which begs the question-"Why does Toyota make them so high off the ground, when people buying midsize trucks are looking for fuel efficency?
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Old 06-08-2021, 10:14 AM   #8 (permalink)
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Quote:
Originally Posted by Vwbeamer View Post
... lowering the truck did drastically improve the gas miliage. ...
I would say it's reasonable to conclude with your evidence that lowering improved your fuel economy, given the magnitude of the improvement. But we can't really know how much (whether it was a drastic improvement) without more rigorous testing. Subtle changes to other factors could be contributing (weather, driving style, routes, and more...)
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Old 06-08-2021, 10:25 AM   #9 (permalink)
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Quote:
Originally Posted by Vwbeamer View Post
Which begs the question-"Why does Toyota make them so high off the ground, when people buying midsize trucks are looking for fuel efficency?
Taller implies bigger size. Sister took huge offense when I called her Toyota pickup a toy truck.
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Old 06-09-2021, 01:45 PM   #10 (permalink)
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variables

Quote:
Originally Posted by Vwbeamer View Post
I think the Cd on my truck is .32.

.32, down from .39 factory, with just a half tonneua cover and lowering.

Using the Aerodynamic and rolling resistance tool, I figure my Cd is .32

Here's the variables I used-

Vehicle weight: 1950.4 kg / 4300 lbs
Crr: .008
Cd: .32
A: 2.7 m2 / 29.5 ft2-Down from stock 29.75
Fuel energy density (Wh/US gal.): 33557
Engine efficiency: .26
Drivetrain efficiency: .97
Parasitic overhead (Watts): 0
rho: 1.225 kg/m3

Varables that I know to be true-
Truck weighs >4300 pounds with me in it
Frontal area stock is 29.75
advertised Cd stock is .39

I took a 762 mile round trip, using 10% ethanol, 87 octane gas traveling @ 70-73 mph ( measured by GPS) I got 27.14 mpg.

Does lowering the truck decrease frontal aera more than just .25 sq foot? I figured that by less tire tread being exposed, because it's now further in the wheeel well.

So is my Cd really .32 now or I'm I tripping.
------------------------------------------------------------------------------------
* E10 reformulated gasoline is 111,836- Btu / gallon
* 32.777 kWh/ gallon
* 6.128- pounds /gallon mass
* A non-turbo'd, non-Atkinson cycle, ICE engine, at steady speed cruise, might be in the vicinity of 36% thermal efficiency, rather than 26%.
* Driveline efficiency will vary with transmission type.
* 92 % driveline efficiency might be more realistic ( if a manual overdrive transmission ).
* 2% accessory losses under the hood.
--------------------------------------------------------------------------------------
* It would be good to have a baseline mpg for the un-modified truck, under the same test conditions.
--------------------------------------------------------------------------------------
* Your mpg and velocity would yield gallons/ hour.
* Gallons / hour yields Btu / hour.
* Btu / hour divided by 2546 Btu/ bhp-hr = Brake Horsepower Hour ( gross)
* Bhp-gross X thermal efficiency = Bhp-hour actual
* Bhp X driveline efficiency = Road Load Horsepower @ test velocity
-----------------------------------------------------------------------------------
* Calculate your aerodynamic- hp from the aero power formula.
* Subtract this from the Road Load -hp = Rolling resistance hp.
* R-R hp = velocity/ torque -to-power factor ( test weight X Cf rr )
* Solve for Coefficient of rolling resistance.
--------------------------------------------------------------------------------------
* With your new ( modified ) mpg, solve for bhp.
* Solve for new road load hp
* Subtract the rolling resistance hp ( it hasn't changed unless you've introduced a significant weight change or different tires )
-------------------------------------------------------------------------------------
* The remainder is your new ( modified ) aero drag power
* Solve for the new Cd using the modified frontal area.
It just falls out of the math.
NOTE : you're presuming a constant BSFC ( the nature of the beast )
The drag could actually be different due to same gearing, moving the engine to a less efficient island on the engine map.
You'll see a trend. Absolute values will be elusive unless measured in a lab. If you're considering coast-down testing, have professionals do it. They'll have the equipment, experience, venue, and expertise to get as good a numbers as are available that way. Wind tunnel?

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