Tonneau Cover Range Improvement numbers

[ZeusDriver]

I can obsess about this stuff... but obviously if I were really concerned about vehicular efficiency, I'd be driving a Prius instead of a Lightning.

So, the theory is this: a tonneau cover lowers the Cd (coefficient of drag) of a pickup truck. That is almost universally true, for reasons that are probably intuitively obvious. The tailgate is an obvious apparent wind catcher. Try to carry a sheet plywood with the flat side facing the wind on a gusty day, and you will appreciate aerodynamic drag.

I've linked a study by SEMA that squares with my understanding of aerodynamics, and especially vehicular aerodynamics.
https://www.agricover.com/downloads/pdf/2007_windtunnel.pdf

Aerodynamic drag becomes increasingly dominant with increases in speed (going up at the square of speed, unlike rolling friction, which stays nearly constant). Power required, therefore, goes up with the cube of speed, (because you are both going faster and pushing harder). Power times time = energy usage. Range increases by the same factor as claimed fuel mileage increases.

If you do a lot of highway driving, (and charge at the robber barons' fast chargers) then a tonneau cover can justify its cost even if you have no other use for it. The range reduction you might imagine from the increase in weight is not measurable, because even the rigid covers are so light. (Do you stick your nose out the window and aim your sneezes backwards to give the truck a little boost?).

My range seemed to improve by about 10 miles (about 5%) after installing my ($100) fold up tonneau cover. However, I was driving fast on the day that I checked, and I don't bother to check on slow driving days, because I know there is no measurable difference. So read the article if you are so inclined, and figure in your style of driving. No "average" gain in range will likely apply to your use case... and my presumed gain won't apply either, unless you happen to drive exactly like me... on the same roads... at the same temperature... in the same wind conditions.

AC and heat have a significant effect on range, as does terrain and many other things. (That is why I used the word "seemed" above -- a definitive, reliable, repeatable test is quite difficult, if not darn near impossible.)

Related: My front air dam extension (the retractable part that extends at 40 to reduce aero drag) fell off a while ago, and I have not been able to detect any difference in range. I tend to drive at close to the speed limit. Maybe if I drove at 20 over all the time, I'd notice a difference.

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[GDN]

Many people obsess about this and most everything in your second paragraph has been proven untrue.

 

[RickLightning]

People can't measure this without instrumentation. The front air dam is more efficient across Ford's entire fleet. Gaining a few points here and there makes a huge difference across millions of vehicles.

 

[Zprime29]

Any difference seen in real world testing are in the margin of error after accounting for external factors (weather, terrain, traffic, etc...). I'm afraid this horse is dead. It's a wash.

 

[Henry Ford]

So, the theory is this: a tonneau cover lowers the Cd (coefficient of drag) of a pickup truck. That is almost universally true, for reasons that are probably intuitively obvious. The tailgate is an obvious apparent wind catcher. Try to carry a sheet plywood with the flat side facing the wind on a gusty day, and you will appreciate aerodynamic drag.

The tailgate is in the giant low pressure area behind the cab. Unless you can induce laminar flow from the air coming off the cab you cannot change the CoD in any meaningful way. A bed topper that connects the top of the cab with the top of the tailgate might reduce CoD. I doubt that's even true. The angle is too sharp to maintain laminar flow. Turbulent air flow equals low pressure or put another way, drag.

 

[ZeusDriver]

Many people obsess about this and most everything in your second paragraph has been proven untrue.

Quote a reliable wind tunnel study that indicates that there is no difference in Cd with a tonneau cover and without.

 

[Heliian]

justify its cost

my ($100) fold up tonneau cover.

That is the cheapest cover I've heard of, good buy.

change the CoD i

Open truck box aerodynamics have been studied quite a bit, so much so that the tailgate is designed as an aerodynamic element. There is a huge low pressure pocket of air behind the cab, the flow over the top of it spills down to the top of the tailgate surface, hence the thick and oddly shaped top on it.

A tonneau cover provides no noticeable change in efficiency as it changes that low pressure area enough it ends up a wash VS the laminar flow.

 

[Henry Ford]

Quote a reliable wind tunnel study that indicates that there is no difference in Cd with a tonneau cover and without.

The link you provided is an 18 year old study by an industry trade organization of wind tunnel drag differences of tonneau covers on trucks that are at least two generations out-of-date. They had to extrapolate theoretical data to conclude there "could" be a 1.8% improvement in fuel economy. Real world data wasn't even considered.

This is easily the most important sentence from that study.

The results of this test are positive for
tonneau manufacturers.

Buy a tonneau cover because you want to keep your stuff secure, out-if-sight, and dryish. You'll be happy. Don't buy a tonneau cover for efficiency because you won't see it.

 

[RickLightning]

All the top names in tonneau covers don't claim any efficiency, because it isn't there.

 

[Henry Ford]

Open truck box aerodynamics have been studied quite a bit, so much so that the tailgate is designed as an aerodynamic element. There is a huge low pressure pocket of air behind the cab, the flow over the top of it spills down to the top of the tailgate surface, hence the thick and oddly shaped top on it.

Exactly. What did tailgates look like in 2007, when this study was written?

 

[ZeusDriver]

All the top names in tonneau covers don't claim any efficiency, because it isn't there.

Worksport is a major player:
https://checkout.worksport.com/blog...tonneau-covers-improve-your-trucks-efficiency

Ford motor company claimed that aero efficiency was improved with a tonneau cover, in this video by Chris Bowe: (see 1:10 in )
Credit to Chris for posting his results, which contradicted his expectations.

The SEMA tests were done in a reliable full-sized wind tunnel and found the same effect across truck brands: about 5+% lowering of Cd.

From my own experience with wind tunnels, I've found that objects quite far downstream from the object under test can have an measurable and significant effect of the apparent lift or drag of the object. So it seems unlikely to me that a bed cover has no affect at all on the aerodynamics of a truck. I'd expect either an improvement or a degradation. Can I personally actually measure the difference on the road? Of course not.

But the surprising consistency in the SEMA tests suggests that there is an improvement. The Department of Energy tests (also finding a meaningful improvement) are most often quoted, although I have not read them.

 

[ZeusDriver]

Exactly. What did tailgates look like in 2007, when this study was written?

This optimized-tailgate-top-theory would suggest that the tailgate top on an 8 foot bed or 6.5 foot bed should look quite different than that for a 5.5 foot bed, due to the difference in angle of attack of the downwash. They certainly look the same to me.

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