The Tonneau Cover Fuel Economy MYTH
Jason Lancaster | Jul 30, 2007 | Comments 34
If you’ve considered buying a tonneau cover for your truck then you’ve probably heard someone tell you that it will improve your gas mileage. They say that because tonneau covers streamline your truck’s bed, they reduce drag improve fuel economy. Too bad it’s a GROSS EXAGGERATION.
Studies have shown that tonneau covers reduce aerodynamic drag. SEMA (the society for aftermarket equipment manufacturers) conducted a study in January 07′ that found tonneau covers reduce aerodynamic drag 4.2-7.8%, depending upon speed. This is the most recent study touted by SEMA, replacing one conducted on a 97′ Dodge Ram nearly 10 years earlier. Interestingly enough, SEMA steers clear of telling us how much fuel savings a “4.2-7.8%” reduction in drag would be. Instead, they say quote: “other tests would need to be conducted in order for fuel economy to be calculated, but it is safe to say that a reduction in drag would improve fuel efficiency for these pickups.”
OUR BS METER WENT OFF when we read that last line. Is SEMA actually saying that they spent the money to test aerodynamic drag on four different trucks, but that they didn’t spend just a little bit more to find out about actual real-world fuel economy savings? Something doesn’t smell right…
Here’s our interpretation of the study:
1) The largest reduction in aerodynamic drag (7.8%) was observed at 85mph. If we’re generous, we could assume that 75% of the engine’s power is being used to overcome the force of drag at this speed (the other 25% goes to tire friction, heat loss, etc.). If drag is reduced by 8%, that would result in a 6% reduction in engine workload. If the truck averages 18 mpg at this speed, then reducing engine workload 6% would improve fuel economy 1.08 mpg. SO, at 85 mph, expect to see about a 1 mpg improvement with a tonneau cover.
2) At a more realistic highway speed of 65 mph, drag is reduced about 5.5%. Using the same math as above but assuming that only half of the engine’s power is being used to overcome drag at this lower speed, fuel economy is improved by 0.5 mpg.
3) At 55mph or below, the fuel economy benefit basically disappears. Aerodynamic drag isn’t significant until you reach speeds of about 55-60 mph. Technically there would be a fuel savings, but it would be small. Less than a tenth of a mpg.
4) Finally, there were 6 different tonneau covers tested. SEMA didn’t disclose which was which, but we’re fairly certain that a fitted, over the rail solid fiberglass tonneau cover gives the best fuel economy results. The cloth/vinyl covers buffet in the wind, and anything that doesn’t go over the rail creates it’s own vortex. Painted to match, these covers cost $700 to $1100 installed.
Now before anyone goes off on us for making assumptions, etc., we did speak with a couple of engineers when writing this article. They emphasized these are rough numbers, and that they could be off by as much as 25%. That means that the best case fuel economy improvement is 1.35 mpg at 85 mph, and 0.63 mpg at 65 mph.
Assuming gas is $3.50 per gallon, here’s how the best case math works out:
Driving your tonneau cover equipped truck at 65mph for 20 minutes per day would save you 0.04 gallons of gas, or about $0.14 per day. Under these circumstances, we figure it will take about 30 years of workday commuting to save enough to earn back the cost of a $1000 tonneau cover.
Drive your tonneau cover equipped truck at 85mph for 60 minutes a day and you’ll save 0.33 gallons of gas, or about $1.16 per day. We figure that’s only 3 years and 7 months of workday commuting to earn back a $1000 tonneau cover. Of course, during that same period, you’ll spend over $13k on fuel. We feel sorry for anyone that has to drive their truck 85mph for 60 minutes a day — the fuel costs for just three years would be enough to buy a nice toy (ATV, boat, motorcycle, etc.).
Bottomline: Tonneau covers DO save gas, but not very much. While the aerodynamic drag numbers sound impressive (4-8% reduction), they don’t actually result in enough gas savings to justify the expense for a normal user. If you buy a tonneau, do it because it keeps your bed dry and secure.
Search terms people used to find this page:
- tonneau cover gas mileage
- fuel economy with truck bed cover
- do tonneau covers improve mpg
Filed Under: Toyota Tundra Accessories
If you take into consideration that a tonneau cover will also protect the stuff you are hauling from rain, snow and sunlight as well as keep it from being stolen it may not be such a bad investment.
If you do regular highway miles you can easily get one of the cheaper covers at least half paid for by fuel savings.
It’s not a miracle but it’s not bad.
Simon
[…] read a post about this research over at Thundra Headquarters
Bottom line:
If you want it….buy it. It isn’t going to save you gas. But it might save you the hassle of filling your cab with luggage cause you just HAD to take the truck on vacation instead of the minivan.
Does the statistic prove that tonneau covers really helps you save gas? I just know that yeah, it reduces drag, but it’s the first time I’ve read about such comparisons.
What about toppers? Wouldn’t they be better than just bed covers if they follow the roofline of the truck?
[…] The Tonneau Cover Fuel Economy MYTH | tundraheadquarters.com __________________ 2008 Crewmax X-SP pakage . 20″ JTI rims BFG AT KO 285/55/20 Carbon fiber kit Toytec 3″ lift w 1″ rear WeatherTech floor liners Flo~Pro muffler Model # FP744 aFe CAI on the way […]
[…] this one The Tonneau Cover Fuel Economy MYTH | tundraheadquarters.com __________________ C.M.T.T.F.F. PROUD MEMBER 2008 Crewmax X-SP pakage 20″ JTI rims BFG AT KO […]
[…] of drag of between 4.2 and 7.8 percent. That certainly sounds good, but Tundra HQ called BS on that pretty solidly, but figured that a 1mpg improvement could be had if the truck were going 85mph, but […]
A topper might be slightly better aerodynamically, I don’t even know, but it’s going to add significantly more weight so the difference would be negligible
Mythbusters went over this with all covers known to be used. Only one increased mpg’s but not by much and that was an air gate and no top. No increase in mpg’s for tailgate down, tonneau cover soft or hard, Cap/topper. The big myth was the tailgate down which actually lost mpg’s. They did the test using a Dodge Dakota.
actually. it’s easier to look at it in terms of miles driven per yr. if you drive 15,000 miles/yr. at 60 mph. (this is high for some-but not for me, i know, you can estimate lower).
savings of .15cents per every 20 miles.
that gives you 15,000/20 miles = 750.
750 x .15 = $112.5 savings/yr
7 yrs, pay off a $700 cover, plus factor in dry clean bed + security is worth to you…
same math with 12,000 miles
12,000/20 miles = 600
600 x .15 = $90 savings/yr
8 yrs, pay off a $720 cover, plus factor in how much a dry clean bed and security is worth to you…
Understand but it was proven that no covers give you extra mpg’s….Mythbusters did it on TV.
I have been doing some aerodynamics research for college credit testing several types of aftermarket (and homebuilt) “drag reducing” products. As far as toppers go, they actually increase the induced drag and therefore the Coefficient of Drag, which would decrease your fuel economy. The ideal shape would be a teardrop, and adding that topper gives you a large, flat rear end (unlike a teardrop) which increases drag (think of the back of a semi here). Not to mention the added weight of a topper over a tonneau cover.
In my tests on both a 2005 F-350 and a 2004 GMC Canyon, the topper that followed the roofline back increased the drag by approximately 10% while a flat tonneau cover decreased the drag by approximately 15% on the F350 and 8% on the GMC Canyon. Whether these would actually translate into any appreciable fuel economy gains, I have not yet figured out. However, in smaller vehicles that are already getting better gas mileage, you are going to see a larger increase in fuel economy for the same drag reduction.
In example, lets say you have the F350 and GMC Canyon and you reduce drag on both of them by 15% and this translates to a 10% fuel economy gain. If the F350 gets 15MPG with no improvements, you would get 1.5 MPG more (minus some for the increase in weight). On the GMC (assuming 22MPG stock) you would get a 2.2 MPG increase.
Also the article says “and anything that doesn
Stephan – Thanks for weighing in. First of all, I’m wondering how you’re measuring drag. Is this a wind tunnel test? If so, I’d love to see some of your data to post here on the site. I think most of your data and assumptions matches the data and assumptions we made in the article, but I’m confused about a couple of things.
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First, I think your statement that “all things being equal, smaller vehicles will benefit more from drag reduction” is a little misleading. While it is true that reducing drag on a small pickup by 15% will result in a bigger fuel economy gain than the same reduction on a big pickup, I would say that finding that much of a gain on a small truck is much harder. SO, in other words, you’re statement is correct, but I highly doubt anyone could find that big of an aerodynamic gain on a smaller vehicle. I also think you should re-evaluate how you’re calculating the overall fuel economy improvement. Since the benefit will only be seen on the highway (see below), the average fuel economy improvement will vary based on how much a vehicle is driven at highway speeds.
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Second, I’d like to think I know a little about aerodynamics, but perhaps I could have chosen my words more carefully. I’m not sure if a “vortex” is created, but I’m quite certain that an over-the-rail tonneau cover has a lower drag coefficient than an under the rail tonneau. That’s the point I was trying to make. My apologies for using the wrong terminology if that is indeed incorrect.
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Finally, I’d like to hear your perspective on the fuel economy calculations. As you well know, aerodynamic drag isn’t usually a significant contribution to fuel economy at less than highway speeds. Therefore, whatever fuel economy boost a tonneau cover will provide will only occur at the higher speeds. Do you think any of the fuel economy assumptions are incorrect?
Yes, this is a wind tunnel test. Unfortunately, it is only a scale model wind tunnel test (my budget was only about $1,000). So I am using 1:18 scale models of several vehicles. The point of my research is to see what ideas and inventions from the aviation world can help the automotive world. So I have been testing different styles of bed covers, vortex generators, belly pans, and wheel covers to see how they affect drag.
As far as it being harder to see a 15% reduction on a smaller vehicle, especially a small truck, is not that hard. In my tests, with the GMC Canyon equipped with an “aerodynamic” bed cover, vortex generators, rear wheel covers and a belly pan, I was able to see a 17% reduction in drag. The F350 identically equipped saw a 16.5% drag reduction. It is going to be hard to see large amounts of drag being reduced, but in a vehicle that doesn’t produce as much drag in the first place, a smaller amount of drag reduction will produce the same percentage reduction.
I don’t know if that makes much sense how I said it. Let’s say we have a vehicle that produces 1000#s of drag. To get a 15% reduction, you will have to lose 150#s. But if you have a vehicle that only produces 500#s of drag, you will only need to lose 75#s to see that same 15% reduction in drag. As far as calculating the fuel economy improvement in my message, I wasn’t actually using any equation, I was just using it as an example.
As far as an over the rail tonneau cover having a lower drag coefficient than an under the rail, even if it does, 2 inches over the bed is not going to cause that much greater drag. The main thing that is going to cause more drag is adding things like a bug shield or a windshield shade. As far as the tonneau, as long as it’s mostly flat, it’s better than the air going down into the bed, and becoming turbulent. A vortex is a swirling column of air but unlike turbulent air, it isn’t always bad to have a vortex. That was the reason for vortex generators, they produce small swirls of air to direct the air where it needs to go. A big usage of VG’s are to keep the air flowing over the ailerons when the wings are at a high angle of attack. To see what I mean by a true vortex view this: http://www.youtube.com/watch?v=E1ESmvyAmOs or look up “NASA Airliner Wing Vortice Tests” on youtube. At first it seems like it’s just making turbulence, but at approximately 45 seconds, you can see the vortex really starting to form.
Vortexes are formed mainly from sharp edges or protrusions. Things like the tip of an antenna should produce some sort of vortex, or the tip of a wing, or the sharp back corners of a camper shell. So depending on the circumstances, a vortex can be a good thing or a bad thing. In the case of the back of the camper shell, it’s a bad thing. When it comes to the vortex generators on top of a vehicle, it prevents the airflow from separating from the vehicle, which is a good thing. It’s odd to think about, but at times, it takes a little turbulence to get rid of a larger amount of turbulence. They are most effective at angles over 15 degrees. I.E. a pick up back window, hatchbacks, SUV’s, etc.
As far as the fuel economy boosts only occuring at high speeds, yes and no. Obviously, they aren’t going to help much in town, but once you get to 55 and steady driving, there should be an improvement. Drag does tend to increase almost exponentially so once you start getting to higher speeds you will see bigger jumps between say 80 and 85 than you will between 55 and 60, but there is no denying that the improvement seen at 55 can be just as great as the improvement seen at 80.
Since gas engines tend to have a efficiency curve that tends to peak at a certain RPM, depending on how the vehicle is geared, the engine may peak at 60 MPH. In this case, the extra drag reduction seen at 80 MPH may not be enough to really see a great MPG gain since the engine is becoming less efficient. On the other hand, if the vehicle is geared to keep the engine at that “sweet spot” while moving at 80 MPH, then you should see a much bigger gain.
Sorry if this is choppy, I’m working on putting a paper together for my Aviation Safety class. I hope I at least made a little sense!
Stephan – Another excellent comment – thank you for contributing. I think your point about powertrain optimization is correct. If the manufacturer optimizes for 80mph, than a drag reduction could have a profound impact on fuel economy (at least at the higher end). At this point, all manufacturers optimize their powertrains for about 60mph. That’s the speed the EPA uses during their fuel economy tests.
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I’m intrigued by your ideas about belly pans and wheel covers, but I’m wondering if you focused only on the tonneau cover, would you see that the larger vehicle (with the larger bed) experienced more of a drag reduction than the smaller vehicle (with the smaller bed)? I realize there’s some proportion to the numbers, but in my mind it’s easier to reduce drag on an 8′ long-bed F350 with a tonneau than it would be to reduce drag on a 5′ short bed Canyon. Has your data shown as much?
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Thanks for the explanation on vortexes as well – I learned something today.
Yes, you are correct about the tonneau cover. When I went back and looked at just the tonneau cover I did only get a 8% reduction in drag on the GMC Canyon. Where on the F-350 model, I got a 16% drag reduction with just a tonneau cover.
Looking back over my smoke tunnel tests (which I did before the wind tunnel just to get an idea of how each modification would affect the air flow around the vehicle), I can’t tell for sure if the length of the bed is the biggest determining factor or not. On either model, the bed is approximately 60% of the full height of the truck, and is approximately 39% of the length. So it may be due to the length of the bed or it may be due to the “scale effect”. Unfortunately, I won’t be able to determine that for sure as I will not be able to do full-scale wind tunnel tests. Overall, though, the Canyon only produced approximately 75% of the drag produced by the F-350 even though the Canyon is 88% of the length of the F-350. So it may have to do with the complete airflow pattern over the vehicle.
The fact that the Canyon is more streamlined than the F-350 may mean that adding the tonneau to the F-350 affects the total air flow more than when you add the tonneau to the Canyon. Or it could be just because there is more area in the bed on the F-350 than there is in the bed of the Canyon. I am going to have to ask my aerodynamics professor to see what his opinion is and get back to you.
Sorry if this doesn’t answer your question as well as you like, but I will do some number crunching and question asking and try to get a more definitive answer back to you.
Stephan – I think your data and my assumptions (and the assumptions of the engineers I spoke with before writing this article) all agree. The fuel economy benefits of a tonneau cover aren’t substantial for most vehicles, and even when the drag is reduced, other factors have more impact on the total fuel economy benefit. Bottom line – buy a tonneau cover because you need it, not because it will “pay for itself.” It won’t.
Agree Jason… I bought Extang Black Max because I wanted a soft tonneau cover to remove when I needed a truck bed. If it was going to pay for itself I would have settled for it to cook me a meal.
Yeah. My results have shown that the best way to reduce the fuel economy is to pick and choose what you add on. As far as the Canyon goes, the best results I saw (approximately a 19.2% drag reduction) was when I used just the tonneau and vortex generators. On the F350 model, the best drag reduction I saw (approximately 18.8%) was when using an “aero cover” and the belly pan. If you used a tonneau the reduction dropped to 18.1%, but that’s still not a bad drop. Of course, this won’t translate directly into fuel economy, especially depending on how heavy the belly pan is, but on the Canyon, the tonneau and VG’s really shouldn’t add that much weight for the amount of drag reduction that they show. I don’t think my vinyl tonneau and frame weigh much more than 30 pounds. And vortex generators probably weigh a pound max for a set of ten. So, yeah just tonneau covers aren’t the best way to increase your fuel economy, but you can’t say that it’s a complete myth. There are just too many variables to rule anything out 100%. I definately really like the way the tonneau looks on my pickup, too! The extra storage space isn’t bad, either.
Stephan,
Have you done any tests with only changing the tonneau cover factor? Are you saying you once saw a 16% change in drag, or was that in combination with other things such as the belly pan, vortex generators, wheel covers, toppers, etc.?
Stephan – It’s true – we can’t make a blanket statement about the fuel economy benefits of tonneau covers in general for every vehicle. HOWEVER, the point here is to dispell the myth that tonneau covers are good for fuel economy. They might be, but in the vast majority of cases for the vast majority of drivers, fuel economy benefits are small and/or nonexistent. Therefore, people shouldn’t buy a tonneau to save gas – they should buy one because they want one.
Just curious. I didn’t really have time to read all of the comments but was there any data on the length of the box on the truck? If the box is shorter with a tonneau cover verses a long box? Not sure there is a lot of difference but the length of the truck should factor in.
Ok I have a 1998 f150 4×4 automatic and it’s rated by fueleconomy.gov at 12-17mpgs. I have made many many modifications to my truck to improve fuel economy and a soft tonneau cover is just one of them. When you are thinking about increasing your fuel economy you have to think about doing small things that add up overall that is the method I have used and have increased my mileage to 16 in city to 27 on the Highway an overall increase of 37%. There is no miracle product that will increase your fuel economy dramatically yet they all claim it. I see a tonneu cover as having multiple benefits and well worth the money (styling, keeps things dry, security and improves fuel economy by 3 – 5%).
Also in the article they talk about tonneau covers costing between $700 and $1100 and that’s just insane you can get a good tonneau cover for between $300 and $600.
Some might be curious about what I have done to improve my mileage so dramatically. So here are my mods listed below.
K&N cold air intake , Edge Evolution CS Gas programmer , Electric fan kit , Underdrive pulley , Amsoil Signature Synthetic oil and transmission fluid, Lund soft Tonneau cover , NGK iridium spark plugs, Magnaflow Catback exhaust .
Brandon – No data on length, but my guess is that long boxes benefited more from a tonneau than short boxes…but when I say “long” I’m saying 8′.
Jason – Fair enough. You’ve got a good list of accessories there, and I think that they all add up to savings. However, only because I like to argue, I’ll say that your results might be exceptional. Most guys don’t get those kind of results…
I agree that tonneaus can be purchased for less than $700, but not painted to match units.
Jason – BTW, thanks very much for commenting. I wish more Ford guys did. Much appreciated. 🙂
[…] tonneau cover gas mileage? The Tonneau Cover Fuel Economy MYTH The Tonneau Cover Fuel Economy MYTH | Tundra Headquarters __________________ Riding through this world, all […]
Your BS meter tripped mine.
Over 60mph 90% of the fuel used in a truck goes to the aerodynamics so yes at highway speeds it’s a substantial savings.
If you extended the tonneau cover out the back of the tailgate, into a cone shape, you would get Hatchback fuel economy!
Spencer – Sorry man, but the math doesn’t back out. The aerodynamic benefits are minimal b/c the biggest factors effecting drag are ground clearance (higher ground clearance = more drag) and frontal area (trucks are bigger). Tailgates are a minor source of drag on all trucks, and even less so on newer trucks. If you drive exclusively on the highway, you’ll get your money back in a few years. Otherwise, it’s a net loser. Buy a tonneau because you want one, not b/c it will save you gas.
Experience with truck bed covers for two pickups have show highway mileage improves significantly with a bed cover.
Something as simple as a tarp (1st truck) made a mileage increase for my Mitsubishi Mighty Max. This increase overcame the mileage cost of the weight of the fully loaded bed. I got better mileage loaded than empty without a cover.
An Aerocover on a Dodge Dakota also improved the mileage but greater than the tarp.
[…] Originally Posted by GhostRam94 my tonneau helped with gas, id say 1-2 better at highway speeds, at 80 i get around 17-18 and 60 im 24+ all day long…. at 70 i average right around 20-21 While a tonneau does help save gas, the savings are very, very minimal. An increase of 2mpg is really a stretch. Studies have proven that at normal highway speed of 65, the fuel savings would only amount to approx 0.5 of a mpg. And it only gets worse at 55-60…the savings are less than a tenth of a mpg. And before anybody questions my post, read the following article… The Tonneau Cover Fuel Economy MYTH | Tundra Headquarters […]
[…] The Tonneau Cover Fuel Economy MYTH | Tundra Headquarters Blog Pay close attention to one of the last paragraphs…. […]
You guys are straight up idiots, yes it helps gas mileage, i put one on my truck and im gettng a couple miles to the gallon better, not much but when you drive 100 miles a day like me it adds up over a month
Thanks!
Why bother to read when you can just name call? If you read more closely, you’d see that tonneaus *can* boost fuel economy, but only for people who do a lot of highway driving, and then only at higher speeds. Am I surprised that you gained some gas mileage? No. But you are the exception to the rule. (NOTE: I’m assuming you drive a lot of highway miles at high speed…you must if you’re going 100 miles a day).