aerodynamics (long)
 

"For a full-size truck, a change in drag coefficient of 0.01 is approximately equal to an improvement in fuel economy of 0.1 mpg on the combined city/highway driving cycle," says GM's Schenkel. "The same drag coefficient reduction can improve a car's fuel economy by approximately 0.2 mpg."

Volvo's Frasher says the force acting against a car by the air it moves is a function of:



Cd x Frontal Area x Density of Air x Speed Squared

Speed clearly is an important part of the equation. At stop-and-go speeds, drag isn't a big deal, but the faster you go, the more it matters. At 70 mph, you've got four times the force working against your vehicle that you have at 35 mph.

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."

It turns out the biggest gains are to be found on pickups — not by dropping the tailgate (a common misconception), but by installing a tonneau cover. "A tonneau cover improves the aerodynamics dramatically — on all pickup trucks," according to Ford's Wegryn. "In general, a tonneau cover can provide a drag reduction of 2 to 7 percent, depending on cab style, box length and overall vehicle Cd. Average fuel economy improvement ranges from 0.1 to 0.3 mpg." From an aero standpoint, it doesn't make a difference if you choose a soft or hard cover.

-http://www.edmunds.com/advice/specialreports/articles/106954/article.html

thoughts? I think its more concrete than most claims myself.

Sound like info from people that know... :)

i found this after reading about a superduty in the newest issue of diesel power magazine that gets 26mpg, the biggest mods he did was lower the truck and build a fastback-like tonneau cover.

I don't remember exactly the grade, but I learned this early in high school, maybe in junior high. This is why driving slower is one very good way to decrerase wind drag, hence increase mpg.

The teardrop shape has the smallest coefficients, so cars with shapes similar to it will have less wind drag. With trucks, especially 18 wheelers, there's no way to reduce the frontal area, but fairings can be used to smoothen the shape, and a fastback like tonneau cover does the same thing for a pickup.

But MythBusters PROVED...

:D

Only to a point. At slower speeds (5~25), wind drag isn't as much of a factor, and you aren't making enough miles per unit time to make up the gasoline overhead of running the engine (unless you have a hybrid and the engine is off).

gas mile@ge vs speed

http://www.fueleconomy.gov/feg/images/speedVsMpg3.gif

-Jim

That chart assumes modern automotive gear ratios, engine torque curves, etc. Back in the heyday of the Mobil Economy Run (1950's and 1960's), 35mph proved optimum, but those were inline sixes with one barrel carbs, torque peaks below 2000rpm, and three speed manual transmissions.

Jim

I think that its the honda insight (hybrid) that is teardrop shaped, but trucks would look really weird shaped like that. also somewhere online i saw some factory testing info on truckers drafting each other and how much that helps, too bad its illegal though, although air diverters and flaps on the back of trailers and some funky looking side skirts helped too.

jimandandy, according to those economy runs my falcon gets over 30mpg! although i dont think thats too far from true with a little work. i need to get it on the road and find out... :-X25

[QUOTE=PSKSAM2]Only to a point. At slower speeds (5~25), wind drag isn't as much of a factor, and you aren't making enough miles per unit time to make up the gasoline overhead of running the engine (unless you have a hybrid and the engine is off).
[/QOUTE]
I forgot to mention that one need to be in the last gear -- 3rd, 4th, 5th, etc., to keep the engine rpm low. And as it was mentioned, the optimum speed may vary significantly, and that mostly depends on gearing, engine, and the shape and frontal area of the vehicle.

check out the thread on the 26mpg super duty in the diesel threads