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Discussion Starter #1
If you don't ever drive over 40, forget aerodynamics. Under 40 MPH, a Scion is just fine.

At 40 the road load equation says that aero drag passes rolling resistance and from there the aero drag HP goes up with the cube of the speed increase. To double your speed, you have to increase HP (and fuel burn) eight times. So trucks that drive the open road really need to look at aerodynamics if they want better MPG.

The aerodynamic component of the road load equation is:

HP = k x Cd x A x MPH^3

k is a constant that converts units
Cd is coefficient of drag
A is frontal area
MPH is road speed times itself times itself

Cd for pickups starts out being about 0.45 to 0.48. A Corvette is the slickest common seen car on the road and they run about 0.30. So if the Cd of a truck could be reduced to say 0.40 (a good reduction) that would reduce drag by 12%.

Area is easy to compute. Width times height, but with one kicker. Double the area between the bottom of the front bumper and grade. For instance, my truck sits 72" tall and is 80" wide. Raw area is 5760 square inches or 40 square feet. My front spoiler sits 7.5 " above grade so I must add in the area under the truck again. I have to add (80x7.5=) 600 square inches so my true frontal area becomes 6360 square inches or 44.1 square feet. You can see how a lift rapidly increases frontal area and therefore drag. Use your tape measure, but SRW trucks are usually just under 80 inches wide. Over 80" wide and they need side marker lights. Duallies measure at the "hips" and is usually roughly 90". Height is the highest point on the cab or cap (if the cap extends above the top of the cab). for a F350 SD, my 72" height represents the fact it has been slammed 4" in front and 6" in back. Measure under-vehicle area from the bottom of the bumper (not the pumpkin) to the road. A unmodified 4x4 SRW will stand about 80" high and 80" wide and have an under-vehicle height of about 14". So the stock area is (80 x 80) + (80 x 14) = 7520 square inches or 52.2 square feet.

As you can see the equation is very sensitive to road speed. There is a Dodge Cummins guy who get excellent MPG simply because he never drives over 40 MPH except down hill. The same truck at 60 MPH needs 3.4 times the HP for aero drag, so no wonder he gets sparling MPG. I can't drive that slow. People in Indiana would shoot you. He would be beheaded with a dull knife in a hammer-down place like SE Michigan.

Note that all pickup sized diesels have roughly the same engine efficiency so more road HP equates to proportionally greater fuel burn.
 

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Discussion Starter #2
Configuration and Coefficient of Drag

Making a vehicle "slicker" is working to reduce coefficient of drag (Cd).

The slickest shape in our Reynolds number range (aerodynamicists talk Reynolds number, not speed) is a teardrop. The worst shape is a 4x8 being pushed sideways through the air. In fact the flat plate is the standard of drag with a Cd of 1.0. In our Reynolds number range the back of the vehicle matters a lot more than the front. (Not true of jets but they operate in a much different Reynolds number regime.) A bluff-nosed truck can get pretty decent Cd, but a flat back end is the kiss of death.

What vehicle designers want to do is to minimize the low-pressure area behind the vehicle. This vacuum literally sucks the truck backward.

For Super Duty sized vehicles the worst vehicles are vans, Excursions and trucks with caps that look chopped-off. These vehicles have enormous low pressure areas behind them and a Cd of 0.55 or more.

Once you get past those configurations, you have the open-bed setup. This si the way most trucks are sold. there is an urban legend that running with the tailgate down gives you a lower Cd. WRONG!!! Endless wind tunnel tests show that running with the gate up creates less drag than running with the gate down. running with the gate down makes the entire area of the bed a huge low-pressure area and then adds the part behind the bumper. With the gate up, the low pressure area is broken into two smaller areas.

The next step up is a flat tonneau. Hard or soft, both work. Here the low pressure area behind the cab is greatly reduced. I put one on my already pretty efficient truck and gained 1.5 MPG. You can tell when you done something right because the bed gets quieter. Try it. Run down the road with an open bed (turn off the stereo - you can't hear this stuff over Willie Nelson), then try it with a tonneau. Much quieter. Remember that it takes energy to make aero noise, and where do you suppose that energy comes from? Diesel fuel burning. Not everyone can live with a tonneau. I see a lot of drive-aways (RV delivery). They have to be duallies with fifth wheels. drive-aways never get back-haul so they are deadheading back on their nickel so they have to be fuel-efficient to be profitable. Many of these trucks have soft tonneau mounting rails to make the run back a little more fuel efficient.

From there we could put on a sloped "fastback" cap or fairing. Back in the 1960s NASA and NASCAR (in the days of seven liter engines and F series tires) researched the optimum angle. They found that 28 dgrees was the most angle you could have and keep the boundary layer stuck to the car. when the boundary layer peels off the car drag is induced. For the average Joe, I would recommend a 25 degree angle because 25 degrees is an easy-to-layout 6:12 slope. My sloped fairing was good for another 1.5 MPG over the flat tonneau. The closer you can get to a wedge or point the better, but simplybreducing the rear low pressure area is very worthwhile.
 

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Dave,

Show us some pics of your ride.....it has been along while since I have looked at it.
 

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Discussion Starter #4

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CSAR Bish
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Well I just removed my cap and put it on my trailer for now, dam I forgot how much pep the truck had before the cap, its a much better increase and Im sure my miliage will come back, I think I also need to disco the batteries to reset the puter since I moved from 15 ft elevation to 6500 ft
 

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Discussion Starter #7
Think about a tonneau, XTrmXJ. They work. Hard or soft, even if only used part-time.
 
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