I stole this from fordf150.net Saw it a while back, thought it was ggood info.

Do Tonneau Covers Really Improve Gas Mileage?
Truck owners often have their own theories on how to improve gas mileage and reduce the air drag on their pickup trucks. One simple solution is to lower the tailgate. Another is to replace the tailgate with a cargo net. Bed caps or tonneau covers can be added as well. According to the Federal Chamber of Automotive Industries, while sales of passenger vehicles in July 2004 were down 9.2% vs. July 2003, sales of SUVs and light trucks increased 1.2% and 4.2%, respectively. With the desire to own pickup trucks combined with high gas prices, it seems as though more studies on how to decrease drag should be conducted.

In the simplest terms, drag deals with resistance to airflow. Cars and trucks alike experience drag. The drag coefficient, Cd, is usually determined experimentally—the lower the value of Cd, the better. The most important thing for the average car or truck owner to know is that the larger the value of Cd, the more work the automobile has to do to push against the air it drives through. And if the car or truck has to do more work, it is going to use more fuel.

Two students from Western New England College’s Department of Mechanical Engineering, Todd J. Ortolani and Vanwijak (Kehm) Ewosakul, wrote a paper in 1997 entitled "Improving Aerodynamic Characteristics of a Dodge Ram Pickup Truck." Their paper describes a series of experiments in which various conditions were simulated in a water tunnel, while making use of different aftermarket products, in order to determine the coefficient of drag for each condition.

The coefficient of drag of the pickup without its mirrors was 0.432—this is the baseline value in order to compare other conditions. Notice that when a square cap was added, Cd had a value of 0.480, representing an 11.11% increase in the coefficient of drag. Surprisingly, there was a decrease in the drag coefficient when the tailgate was lowered, with a value of 0.414, even though the drag coefficient experienced an increase when the tailgate was completely removed. Cd was reduced by 11.81% with a value of 0.381, when a tonneau cover was added to the truck. And finally, the best result were produced from a cap Ortolani and Ewosakul designed—the drag was reduced by 30%, with Cd equal to 0.302.



Condition Drag Coefficient, Cd % Change From Baseline MPG
Baseline 0.432 N/A 16
Square Cap 0.480 11.11% 14.9
Cargo Net 0.461 6.71% 15.33
Removed Tailgate 0.447 3.47% 15.65
Lowered Tailgate 0.414 -4.17% 16.42
Tonneau Cover 0.381 -11.81% 17.18
New Cap Design 0.302 -30.09% 19
Source: Improving Aerodynamic Characteristics of a Dodge Ram Pickup Truck

In order to determine how coefficient of drag translates into fuel efficiency (MPG), a simple equation is derived. First, assume the Dodge truck is equipped with the 5.2 liter V8, combined with an automatic transmission. If the truck is 2WD, an average 16 mpg fuel economy is assumed. Under this condition, the increases and decreases in drag can be equated to fuel consumption using a simple equation. We will call Y the baseline mpg, which is 16. X will represent the ratio of the change in mpg (A) to the change in drag (B). Z will represent the percentage change from the baseline condition to a new condition, where a decrease in drag is a positive (%) and an increase in drag is a negative (%). In each case, A is what needs to be determined, and it can be found by: A = ( X x Z ) + Y. The results in the table above show the gas mileage results using this equation.

The study indicates that the best way to reduce drag, and thereby improve fuel efficiency for pickup trucks, is to add a tonneau cover, unless an improved cap design is developed. However, studies have indicated that other factors can also contribute to improved fuel efficiency, including choices in air filters, aftermarket grilles and by keeping tires properly inflated. Also, it is important to remember that drag depends on the size and shape of the vehicle, and therefore a Chevrolet, a Dodge, and a Ford pickup with the same cap might have different drag coefficients because of different design features. While the increases or decreases in drag coefficients might not mean much to most truck owners, increases or decreases in fuel economy have both economical as well as environmental significance.

For more information on this study, contact Megan McKernan at SEMA, meganm @ sema.org, 909/396-0289, ext. 125.

Source: SEMA Research and Information Center. Used with permission.

 

This was the last post I see. I dont quite get what you just said....

Anyhow, I agree with SAJEFFC. I went from 15.5-16 mpg with 70-75 highway driving to 17 by slowing down a bit. Plus, I hear that the speed limit was 55 for the longest for fuel conservation, not neccessarily for safety, although there where probably different motives for this.

 

Good post..thanks! But all that math stuff gave me a headache.

 

The speed limit was at 55 for political reasons. Safety was never mentioned. Even gas mileage was not the actual reason, any more than gun control has anything to do with crime.
The votes were there for 'doing something' whether or not it worked/mattered never entered into it.
. Power bands, drag co-efficient, etc. change all the rules.
And gas consumption went up during the 55 mile campaign in the big cities, due to simple math. N amount of cars VS Q amount of time to traverse L amount of freeway. When the speeds are kept down, there are too many cars (TMC) to funnel through the system unless the cars are moving at a fast rate.
Every study showed the congestion was caused by ‘bottle necking’.
They raised the speeds, and it took an increase of 30% more cars before they started having the bumper to bumper stuff again.
Even now, it's not as bad as it was in the '55' days.


All the safety talk came after the gas prices went down. The 55 limit was actually a way to give the feds complete control over the states budgets. Read or watch the debates leading to the end of the 55 law. The Democrats were willing to raise the federal limit to 150MPH, if it still left them in control.
Safety, huh?


Junk information still raises it's ugly head on the tonneau, tailgate removal issue.

Drag coefficients apply only when the EXACT truck and road conditions prevalent. The information these studies find are for 1 truck, 1 model of that truck, 1 area, 1 set of conditions.

Take time to read them. Even the best are WAG's at what your conditions could be.

The best drag coefficient can be obtained by taping clear plastic over your bed (simulating a perfect fit tonneau.) and taping up the undercarriage, where about half the drag occurs.

This will deliver better mileage than any other method. Tested by more than a dozen sites. Google for pickup mileage study.

Leaving the bed down will either get you better or worse mileage, but it's depending on the air speed you attain.
If you go slow, air won't 'pocket' in the bed. But any drag improvements will be very small anyway. At higher speeds air will pocket in the bed, 'smoothing' the air flow out. At even higher speeds, the drag will increase. So your benefit will come and go as you change speeds. Wind speed that is, which may have little to do with ground speed.

A removed tailgate will probably get better mileage around town, but so little you could never measure it. And worse on the freeway at 65. In a Schlieren test you would be 'dragging' a tail of air at every sharp point in the mechanism, and gaping holes.

A lowered tailgate, depending on the shape of the bed, or the shape of the tailgate, will either cut or increase the drag. and that is speed dependent.

I used to lower the tail gate in a 1969 Ranchero I was using to freeway 44 mile each way to work. I always got better mileage with the tail gate down. 1-2 mpg better. I would not expect the same thing to happen with a different body style. It might work, it might not. But if it mattered to me, I would test it. In no case would it exceed the gains from a cheap tonneau, tightly secured.

The tail gate nets don't seem to help. They did on a boxy Toyota I used to drive in the 80's, but on a 2001 Dodge Dakota we just tried one on, it made 0.0 difference, mixed driving, three months test both ways. It cost 65.00 dead loss for the trucks owner...

I reiterate, test information is only valid for your truck body style, and your conditions.

Using a vacuum gauge or a Scanguage, or any kind of instant mileage monitor, and using that feedback to understand the vehicle is the best way to improve mileage in any vehicle, and this truck more than any other I have driven.

As to the makers really caring about mileage, it's sort of true.
They only care about sales. Mileage matters, so long as the truck/car drive nicely, looks great, and can be sold for high profit.
If gas mileage is on the customers mind, then that is important to the maker. It hasn't been important for a long time....

To beat CAFE limitations, there are easier methods. Note Cadillac dealers always have a cheap import badged as a Caddy. That's to lower the fleet miles.

Trucks are set high enough that as long as no maker rocks the boat, they don’t need to make any changes.

Notice all the big light duty trucks get about the same mileage.... Rangers, to Lariats, same at all brands. Differences are minimal.
More than 12 less than 20. If they really were concerned, there would be a bigger spread.
Chris

 

and they need all the help they can get!

 

If you leave the tail gate down the sides of the bed will shake and crack the bed! I have seen it before. Leave the tail gate it up it aids in strenght. The tail gate down is for old trucks, not new ones that have been tested in a wind tunnel.

 

It doesn't work well for the old ones either. I've seen a lot of old ones that the bedsides get to flopping and metal fatigue causes the metal to fail and it ruins the entire bed (it takes it a while for it to show up). It is most evident on construction and farm trucks.

 

At work, our company has a 1994 Dodge Dully truck that has been without a tailgate since the company brought it (1995) You can seriously stand right next to it's bed, put ur hands on it, and shack the bed with your hands and hear and see the bed jumping back and fourth! It's extreamly loose!