Tailgate up or down?


#1

When the caller asked whether a pickup’s tailgate up or down gives better fuel mileage, Tom and Ray didn’t hesitate to state that tailgate down is best. The so-called “reasoning” in Tom’s words were with tailgate up “you are carrying around a ton of air.” BOOOOGUS!
The turbulence patterns are such that tailgate up is actually more fuel efficient.

A recent Mythbusters show did this testing for five conditions:
Tailgate up
Tailgate down
No tailgate
Hard cover on cargo bay
Mesh tailgate replacement - they guessed this would give lots of wind resistance and worst mileage.

The results, in order of best to worst:
Mesh cover - apparently turbulence pattern generated is lots of small vortices instead of a large one, and less drag.
Tailgate up and Hard cover about equal
No tailgate
Tailgate down - worst.


#2

Yes, after they ran that segment the first time, they got a call from a Detroit auto engineer who debunked the tailgate down myth. He told them that with the tailgate up, a bubble of recirculating air is created in the bed when the truck is moving, and the recirculating air bubble helps direct the airflow smoothly over the back of the truck, decreasing drag. Dropping the tailgate cause the recirculating air bubble to collapse, and increases drag.

It’s been proved. Dropping the tailgate increase drag.


#3

Over the years I have owned quite a few Ford 1/2 ton pickups and vans with the 4.9 engine and driven them many hundreds of thousands of miles and the van body seems to give a significant improvement in fuel economy over pickups and a topper on a pickup seemed to have no effect on fuel mileage while odd shaped cargo in an uncovered pickup would often drastically reduce mileage even though the cargo did not extend out of the draft of the cab. I would enjoy seeing the Mythbuster episode mentioned.


#4

The Mythbuster show was Episode 64. This was a more inclusive revisit of the simple tailgate up vs down test.
While copyright issues prevents a YouTube showing of the episode, here is a link to a text description of the show.
http://kwc.org/mythbusters/2006/12/episode_64_more_myths_revisite.html
Scroll down to the middle of the page to see the tailgate test description.
Everyone was surprised by the mesh tailgate actually improving the mileage.


#5

The truck was getting 25 mpg at 25 mph and 10.5 mpg at 55 mph. What make and model truck was used?


#6

The tailgate up vs tailgate down Mythbuster results are surprising. I’d have guessed mpg would be worse w/the tailgate up. Like Tom and Ray said. Just shows that what seems like it should be true by common sense intuition isn’t always so. I’m wondering though if there are actually two effects: (1) the bubble idea mentioned above; and (2) when the tailgate is down, it is now partially blocking air flowing under the truck.


#7

There have been several tests (including wind tunnel evaluations) that confirm tailgate up is better than down, or removed.
Here’s GM’s comments, including that the mesh versions are even worse than down (I remember that from another study, too).
http://media.gm.com/content/media/us/en/gm/news.detail.html/content/Pages/news/us/en/2013/May/0516-gmc-pickup-aero.html


#8

It appears the Mythbuster folks did the scientific testing and tailgate down is the worst you can do. Apparently the tailgate down causes a lot of downforce and that causes rolling resistance to increase so mileage worsens.


#9

Wow. I never would have thought the mesh tailgate would be the most efficient.


#10

The GM guy said the mesh was pretty bad: “Bloch also advised against nets covering the back of the truck. “Replacing the tailgate with an aftermarket net is worse than having no tailgate at all,” Bloch said in a statement. “Imagine dragging a solid object and a fishing net through water. The net is going to require more muscle.””

Now we can start a thread on ‘Mesh Tailgate - Salvation or Damnation?’


#11

The GMC engineer quoted in the article says the mesh tailgate is even worse than no tailgate (ie tailgate down). But the Mythbusters say the mesh tailgate gave them the best result? Someone is wrong, and I don’t know who for sure. But given the extensive wind tunnel testing that the car companies do, they have tons of data. I’d trust the GMC engineer’s data saying that the mesh tailgate is worse.


#12

You can NOT conclude much from ONE mythbusters test. Sorry…but that’s not very good science. You need multiple tests with multiple vehicles in many different driving conditions to make any type of scientific conclusion.


#13

Here’s some better science. This was documented in a SAE paper in 2004, and summarized in Road & Track magazine (note the comment on mesh tailgates):

"Investigators at the National Research Council of Canada have determined that pickup truck aerodynamics is generally degraded — not improved — by the often-seen practice of lowering or removing the tailgate. Drag is generally greater and, to the detriment of yaw stability, rear lift can be increased by as much as 60 percent. Popular mesh tailgates worsen aerodynamics of these vehicles as well.

The researchers measured drag, lift and yaw behavior of pickup trucks in the same wind tunnel where, years ago, we evaluated Champ-car aerodynamics (“Putting the CART Before the Wind,” June 1984). They also performed Computational Flow Dynamics analyses of two simplified pickup shapes. These CFD results agreed with their tunnel findings.

The differences in CD were measurable, though not profound. For instance, with a 2002 Ford F-150 Crewcab, its tailgate-up CD was measured at 0.5304. Tailgate down, it was 0.5425; tailgate off, 0.5596.

Indeed, there was one case, a 2001 Ford F-150 standard cab, where lowering the tailgate actually improved CD, albeit insignificantly, from 0.5230 to 0.5215. (And, yes, pickup trucks are bricks aerodynamically.)

In their report (SAE Paper No. 2004-01-1146), the researchers discussed underlying reasons for the phenomena. In general, there’s a wake immediately behind any pickup cab, and this suction is greater without a tailgate in place. True, presence of a tailgate adds a small drag increment; but the cab wake predominates.

A partial tonneau cover helped, lowering the extended-cab F-150’s 0.5304 to 0.5072. A full tonneau cover was better, dropping the standard-cab F-150’s 0.5230 to 0.4967."

Case closed, as far as I’m concerned.


#14

Texases, good find.

Verdict: mesh tailgate is guilty as charged!


#15

I’d rather drag a fishing net through water than a flat piece of metal oriented vertically. Wow. That GM guy must be a complete idiot. Even if he is right, his example doesn’t make his point.


#16

I listened to your program while driving across Texas on Saturday. The woman caller was right. Her husband and you guys were wrong about pickup truck tailgate aerodynamics. Tail gate up is more efficient. The bubble of air created by having the gate up allows for smoother airflow of the high speed air traveling over the truck. The links below describe the physics in more detail. And those tail gate replacement nets you mentioned are the worst. They are simply parachutes. Worst then having the tailgate down.

http://truckingtimes.com/articles/pickup-truck-tailgate-up-or-down-aerodynamics-tested

http://www.trucktestdigest.com/TTDfeatures/FeatureTailgateUporDown.htm

http://media.gm.com/content/media/us/en/gm/news.detail.html/content/Pages/news/us/en/2013/May/0516-gmc-pickup-aero.html


#17

Consumer Reports also just weighed in with a test of the 2013 Dodge Ram V8. Results at 65mph, from best to worst gas mileage:

  1. Tailgate up (22.3 mpg)
  2. Tailgate down (21.5 mpg)
  3. Soft tonneau cover (21.4 mpg)

http://consumerreports.org/cro/news/2013/08/pickup-truck-tailgates-and-fuel-economy/index.htm


#18

Well Tex, that report measured CD but did not take into account the weight of the tailgate! The mesh weighs a lot less and that is why there is a disagreement with the Mythbusters results.


#19

Mythbusters is on pretty shaky ground; scientifically speaking.


#20

I’ll take hundreds of hours of wind tunnel testing on many different vehicles over a Mythbusters episode where they do a few runs on one vehicle with much less instrumentation and much less control over variable conditions.

And nothing against Mythbusters, I’m a fan.