Quote:
Originally Posted by Vwbeamer
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 ft2Down 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 @ 7073 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 nonturbo'd, nonAtkinson 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 unmodified 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/ bhphr = Brake Horsepower Hour ( gross)
* Bhpgross X thermal efficiency = Bhphour 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.
* RR hp = velocity/ torque topower 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 coastdown 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?