Tonneau Cover and MPG

[Thomas Rogers]

RichardL,



I doubt that the bed of the ST is so short (or other aerodynamic factors such) as to make the air coming from over and around the cab to never hit the top of the bed. I doubt that seriously.



IMHO, it would be a good assumption and bet that SOME amount of air his hitting the top of the bed.



Anyone got any wind-tunnel videos of the ST or similaly short bed pickups?



The linked article talks about deflectors added to the '07 to cut-down on wind rushing between the cabin and cargo box, entering from beneath and the sides. Clearly if you have a cab that is teardrop shaped and wind is coming from the bottom and the sides, it's probably coming in from the top too.



I will admin that my '04 ST had a kind of "calm" area directly on the surface of the rear window where there seemed to be an equilibrium of sorts...rain used to bead up and dance on that back window...the kids were facinated by watching a few drops of rain come together and dance around on the surface of the window. But how far out past the window that bubble existed is an interesting question. I doubt it extended to the end of the bed.



TJR

 

[Richard L]

TJR,

You said:

I doubt that the bed of the ST is so short (or other aerodynamic factors such) as to make the air coming from over and around the cab to never hit the top of the bed. I doubt that seriously.



IMHO, it would be a good assumption and bet that SOME amount of air his hitting the top of the bed

We are not just talking about an inch or two difference in bed length....A typical full size truck has a 6 foot long bed with some are as long as 8 foot. Compared to the Sport Trac's 4 foot long bed, they are 50% to 100% longer. Aerodynamicly, that has to make some kind of difference, just like the Mythbusters testing of the dangerous practice of drafting behind a big 18 wheeler to improve your gas mileage. The closer they got tucked in behind the truck the better the gas mileage improved.



The tailgate on a pickup truck is just like a car in the draft of the vehicle in front and the closer he gets the less air tubulence there is. I think the same applies to the Sport Trac. The short bed brings the tailgate closer to the back of the cab and changes the airflow. I'm not saying that no air is going into the bed, I just don't think there is as much air going into the bed as there would be if the bed were longer.



This is not a scientific windtunnel test of the Sport Trac, but what put me on to this theory occured years ago when I left my 2001 Sport Trac parked in my driveway one fall night and it accumulated a layer of dried leaves in the bottom of the bed. I assumed that they would blow out once I started driving at highway speeds about 50 mph. I was a bit puzzled that when I did not see leaves fluttering around behind me in the bed, and when I returned from my 20 mile trip, vertually all the leaves were still in the bed? I did not necessarly think they would all get blown out, but I expected that most of them would, but they did not.



I did not think much about that until I got my tonneau cove, just before Thanksgiving, I had expected to see an improvment in gas mileage. Again I was confused when there was no real change that I could attribute to the tonneau cover. Early surveys on this site clearly indicated that nobody was seeing any mileage difference with or without the cover.



If Mythbusters can see a difference in mileage between tailgate up and tailgate down, why can't we see a difference in mileage between tonneauc cover and no tonneau cover?? Rather than explain your theory on that point, you just choose to ignore it



Even if the Mybusters had tested the F150 with and without a tonneau cover, I'm really not sure that those results would apply similarly to a Sport Trac...just because of the bed length and the possible differences in the aerodynamics between the two vehicles.



...Rich

 

[Jimmy Lavoie]

I'd like to add, if there is any advantage to mileage with the OEM cover, it's certainly lost by the added weight.



I think it's a wash when talking about the ST only.



My 2 cents.

 

[Thomas Rogers]

RichardL, regarding your leaves experiment, I assume you had the tailgate up?



Try this experiment:



1. Take a dry, empty 2 liter soda bottle and a small crumpled up ball of paper about the size of a gree pea.



2. Set the soda bottle on its side and set the small wad of paper in the neck of the bottle, just inside the neck that is.



3. Try to BLOW the pea-sized piece of paper into the bottle.



Variant: Put the wad of paper inside the bottle, past the neck and try to blow air into the bottle so the paper comes out.



Results: You can't blow the paper wad into the bottle (or in the variant case out of the bottle).



For that same reasons the leaves don't blow out of the bed.



With the tailgate up any air that is trying to be forced downward into the bed has to displace air already in the bed in order to enter. If the angle/force of the air is such that the air trying to go in won't allow any air to escape, then no air (or leaves) will exit the bed. That is why the leaves stayed where they were.



That scenario is more likely, IMHO, than the aerodynamics of the ST putting an "eye of the storm" bubble over the box.



TJR

 

[Thomas Rogers]

RichardL also said:

If Mythbusters can see a difference in mileage between tailgate up and tailgate down, why can't we see a difference in mileage between tonneauc cover and no tonneau cover?? Rather than explain your theory on that point, you just choose to ignore it

Who said we can't? I'm not going to try to explain someone elses unsubstantiated assertion.



Has anything scientifically been done to prove MPG with T-cover on and off?



I don't think so.



I'm just stating that drag should be less with the T-cover OFF based on common understanding of truck aerodynamics. That should lead to better MPG because of LESS DRAG and LIGHTER WEIGHT...both together are probably marginal. To see the results would require some accurate scientific measures. It's probably not a big deal either way, but I never claimed it was a big difference.



TJR

 

[Richard L]

TJR,

The Mythbusters noted several miles per gallon difference between tailgate up and tailgate down and that is something that does not require a lot of scientific instrumentation to show.



Dozens of members here reported that they did not detect any measurable change in gas mileage with or without the cover. Yes a few claimed improvement with the cover while some others reported reported a slight loss in fuel mileage. I agree that is not exactly scientific proof, but the majority saw no diffenece and will accept that since I got the same results with my Sport Trac.



I find it odd that you would require such scientific proof like the Mythbusters test on the F150 and then require no further proof to proclaim that the aerodynamics on all trucks are the same and so that rule applies to all trucks regarless of their design, shape or length of the bed?



Then you are saying that same rule would apply to all trucks, and a truck with a 2 foot long bed, or even a 6 inch long bed would still form an air bubble and would get better mileage with the tailgate up than with the tailgate down? It appears to me that you are too intellegent to really believe that, but you are simply arguing for the sake of arguing.



...Rich

 

[Thomas Rogers]

Richard,



Was it several miles per gallon difference in the Mythbuster test, or several miles per TANK? I thought it was the latter. I thought the truck with the tailgate up went on to go a few extra miles around the track after the first ran out of gas.



I never said that all trucks have the same aerodynamics. I simply said that I suspect that the bed of the ST has air blowing down on it when moving at highway speeds. Your experiment with the leaves, that you mentioned, re-enforces that opinion as it was that air that kept the leaves held suspended.



Talking about a two foot long and a ridiculous 6 inch long bed is funny but not too useful.



Tell you what...go put your T-cover on, put a few dry leaves, or maybe some dry sand on top of the cover and then drive down the road and let me know if the stuff flies off.



If it does than that's a pretty good case that there is air running, pushing down, along, and deflecting against the top of the t-cover and that the bed is not in some aerodynamic "bubble" of calm air. If the leaves/sand stay put, then I will believe in your notion that the ST has some sort of calm vacuum behind the cab that extends to the full size of the short bed.



To make it interesting, I'll send you a free set of leather "behind the seat" storage pockets, for behind the back seat of your ST IF the stuff stays put.



I would do the test myself, but I don't have an ST any longer.



TJR

 

[Thomas Rogers]

RichardL said:

The Mythbusters noted several miles per gallon difference between tailgate up and tailgate down

Actually, you are wrong. The mythbusters episode showed ONE (1) mpg difference between the two.



Mythbusters facts:

Test Setup

Adam and Jamie were each given identical, new model pickup trucks. They both had equal mileage, same tire pressure, and 30 gallons of gas. Jamie drove with the tailgate up and Adam the tailgate down.



The rules:

-They have to maintain the speed limit

-All acceleration must be done by cruise control

-No drafting

-Windows up, A/C must be exactly the same in both cars



The Test

After 300 miles there didn't appear to be much difference in the gas consumed, but after 500 miles Adam (tailgate down) ran out of gas. Jamie made it another 30 miles before he ran out of gas. This result was the exact opposite of the myth



Water tunnel visualization

According to the experts, a circular pillow of air forms behind the cab of the truck when the tailgate is up. This "separated bubble"/"locked vortex flow" keep the faster moving air from contacting the truck, and thus reduces drag. With the tailgate down, the bubble breaks down and is no longer able to keep the fast moving air out, increasing drag.



In their scale model with the water tunnel, they were able to see that the increased drag. With the tailgate down, the particles in the water were dropping down and hitting the tailgate.

The differences in total miles driven were 500 vs 530, and that is for 30 gallons each. That's a difference of 16.67 mpg and 17.67mpg.



So, as for your claim that people here aren't noting a difference between cover on and off, I submit that most won't notice a 1mpg change. It's awfully hard to show or notice 1mpg savings in the real world as our driving conditions, routines and driving actions change day to day.



TJR

 

[Nick Barber]

My 2c



tonneau in city driving = more weight to start and stop = poorer fuel milege



tonneau on highway = less resistance at constant speed = better gas milege

 

[Thomas Rogers]

Nick B,



I would agree that with your first assertion, that for stop and go and in slow speeds the extra weight is the bigger factor, and the aerodynamics a lesser factor.



And I would agree that at highway speeds aerodynamics is the bigger factor. If we are to assume that the ST bed has wind running across its top at highway speeds, then running with the t-cover OFF provides less resistance (see "Water Tunnel Visualizations" in the quote of my post directly above) AND lighter weight..thus the double whammy of savings and better gas mileage....still probably only a blip in the mpg rating.



TJR

 

[Richard L]

TJR,

You are correct about the Mythbusters test only showing a 1 MPG difference on the F150. I thought it was more. I also agree that 1 MPG is fairly insignificant and can't be attributed solely to the tailgate or tonneau cover.



My point to you is that the short length of the Sport Trac pickup bed could alter the aerodynamics enough so that perhaps the air bubble might not form or provide the same aerodynamic advantage.



I also disagree with your assumption that all pickup trucks would exhibit similar aerodynamic principles as the Mythbusters F150 test. I would suspect that most trucks of that size and with the same or similar length bed would have similar results, but I would also be willing to accept the unexpected. I'm sure most people were as suprised as the Mythbusters were when normal logic would indicate that the truck should have gotten better mileage with the tailgate down?



I think that is where our opinions part ways. You assume that the Sport Trac with the short bed will have similar aerodynamic characteristics as the F150 with the longer bed. Until someone actually conducts an accurate scientific test on the Sport Trac we will just have to agree to disagree.



...Rich

 

[Bill Barber]

Rich/ TJR

Why not take this thread to PMs (do we have PMs?) okay emails, the two of you are eating up bandwidth on a really stupid subject.

 

[Thomas Rogers]

Bill Barber...don't like it, don't read. That's your perogative.



If find your notion of what is scarce to be absurd and bordering on the stupid, but do you see me mention it...oh, guess I just did. Have fun hating.



TJR

 

[Thomas Rogers]

Richard L,



You said:

My point to you is that the short length of the Sport Trac pickup bed could alter the aerodynamics enough so that perhaps the air bubble might not form or provide the same aerodynamic advantage.

could, perhabs, might...



Granted, I don't expect the exact same aerodynamics. But I do expect wind to race across the top of the t-cover. I expect that with a very high level of certainty. So much so I gave you a wager.



So I guess you won't be putting dry leaves on the top of your ST T-cover and seeing if they blow off when driving down the highway?



You made a statement early on that you didn't think the aerodynamics were similar based on the short bed. Now you are saying they might not be. I never said they were the same, just similar. I submit they are similar enough that there is drag on the top of the T-cover, and that's enough for the bubble-effect shown by Mythbusters to apply. I'm assuming that with an extreme high level of certainty.



You can prove me or you wrong easily.....GO FOR IT!



TJR

 

[Richard L]

TJR,

I said we should just agree to disagree. You continue to want to pick apart my statements based a the meaning YOU are reading into my words. I use "Should, Would, Will, Could and Might with a lot of interchangeability. Miy position has not changed and I still do not feel that the aerodynamics of the Short Bed Sport Trac are similar enough to the F150's longer bed that the Mybusters should be assumed to have similare results



You asked me to prove it, and since I don't own a Sport Trac anymore I can't run my own scientific test and why bother since you would consider any results that did not fit you preconced notion as unsubstanciated. Remember, you are the one who originally disagree with me, so I submit to you, that if you think the issue is that important, that you prove ME wrong. Remember that I said that there was no significant difference in gas mileage with or without the tonneau cover on the Sport Trac, and that was based on my theory that the short bed allowed more air flow to pass over the bed rather than into it and the tonneau cover made little if any difference.



You then jumped in with the assumptions that all trucks would create a similar air bubble in the bed as was demonstrated by the F150 tested by the Mybusters.



Since the Mythbuster's test only netted a 1 MPG difference, it surely is not worth arguing about. So stop arguing about word symantics and prove ME wrong on the issues.



You reminde me of a political candidate who snipes at the other candidate on what they meant by one word in a long speach when you know exactly what they meant. Words have very different meanings when taken out of context or in short sound bites. You use the issue as just a starting point so that you can continue the conversation and find more words definitions that you want to agrue about.



...Rich

 

[Thomas Rogers]

Richard L,



Sorry that you seem to be taking this so personally. I'm not sure why. Frankly I think you made it personal from your first post. I was offended by it.



Just indulge me for a second and you will understand why.



My main reason for responding in the first place was because of your matter of fact attitude about telling people what they can and can't assume. Simply put, it is arrogant and it has no place in civilized discussion.



Your original statement was:

So until Mythbusters tests the Sport Trac with and without the tonneau cover, you cannot assume that it will react the same way that the F150 did.

I can *assume* whatever the hell I want, thank you!



Life is filled with assumptions and predictions of what might be based on what we know and have experienced. That is what we do in life. Telling people they can't assume things in this way is futile.



If you disagree with someone's assumptions tell them in a constructive manner why you think they are bad assumptions.



My original statement (that there is more drag with the cover on) was based on one single assumption and the MythBusters segment.



My Assumption: That an ST driving down the road has wind racing behind the cab and across the top of its bed (either covered or uncovered).



To me, that is not a reckless assumption. I'll show how the Mythbusters ep fits in below, but for now, let's focus on that one assumption that you say I should not make. I owned an ST. I had pretty heavy stuff left sitting on top of the cover by accident and it blew off while driving. It's not a reckless assumption, and no one should be chastised for assuming that the aerodynamics of the ST is such that wind blows across the top of the t-cover.



I submit further challenges (to anyone here):



1. Put a baseball cap on a T-cover and drive down the road...does it blow off?



2. Open the rear window, stick your arms out, lay them on the middle of the t-cover and try to light a match when driving down the road...can you do it?



3. Ask 10 reasonable people to guess the results to the (1 and 2) challenges above, whether they own STs or not and see the answers.



I submit 10 out of 10 will predict that the baseball cap flies off and that you can't light a match. Their predictions will be based on assumptions. More than likely these people would be right. This all means that one CAN assume. Which was really my only reason for posting in the first place.



Furthermore, I wasn't saying that an ST and an F-150 are identical. I raised the Mythbusters segment as information to further my point. In that show, so-called experts stated that:

According to the experts, a circular pillow of air forms behind the cab of the truck when the tailgate is up. This "separated bubble"/"locked vortex flow" keep the faster moving air from contacting the truck, and thus reduces drag. With the tailgate down, the bubble breaks down and is no longer able to keep the fast moving air out, increasing drag.

I doubt those experts were talking only about the F-150, but instead were talking generally about all trucks with boxes and tailgates. Am I sure of that? No, but again, it's a good assumption. The only way that a truck wouldn't display such dynamics is if its aerodynamics are such that NO moving air forms behind the cab of the truck...which I simply cannot fathom for any pickup truck on the market.



You've already stated that you thought the ST was one such truck due to your leaves "experiment" and I gave you a credible, physics reason why the leaves stayed in place.



We can agree to disagree, but frankly, IMHO, I've given you very little to disagree with. I made one assumption which seems very reasonable and linked it to a description of a physicial phenomena described by experts and one that I hav

 

[Les]

I had pretty heavy stuff left sitting on top of the cover by accident and it blew off while driving.

How do you know it didn't vibrate off ...

 

[Thomas Rogers]

Good question, Les. It could have.



Try the match experiment, Les. See if you can rest your hand on the t-cover with your arm ou the rear window as someone else drives the ST down the highway on a calm day. See if you can light the match, then get back to me.



My common sense and experience tells me that there will be some wind back there and it is safe to assume such.



Man, I just hate being told that I "cannot assume!"



TJR

 

[Les]

TJR, but remember, when you assume, you make an ass, out of u and me...

 

[Thomas Rogers]

Les,



Funny.



As I said above several times, life is filled with assumption. I assume when I leave my house in the morning that I will return. I don't know that I will, but based on past experiences I assume that I will. I assume that my car will start. I assume that my garage door will open. All of these things I assume will happen, and I expect them to happen.



I assume that gas will continue to rise in price. I assume that it will continue to become more and more scarce over the next half of this century.



I assume that if something has happened before, that when I see similar conditions that it will likely happen again.



Our life is filled with assumptions. It's man's ability to assume and to predict things that make us different than most animals. Most animals act on instinct alone, and very few can actually learn and intuit. We as humans can. We should never be admonished for it.



TJR