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Old 03-01-2019, 11:59 AM   #1
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Truck Box Caps: Do they help with drag

After fighting headwinds I am thinking of putting a cap on my Silverado to hopefully direct the wind a little higher up the front of the toy hauler.

Has anyone gone capless to capped and seen a measureable difference?
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Old 03-01-2019, 12:05 PM   #2
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Ahhhh the cap question. Asked and answered numerous times, just pop it into the search window. Short answer: a 'cap' may result in a negligible mileage increase. Tonneau covers seems to decrease mileage a bit. Those studies are truck alone, not towing.
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Old 03-01-2019, 12:27 PM   #3
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This is just my opinion, but having been in the aviation field for the past 30 years, I might know just enough to be dangerous...


But I would say that I feel that it should make some level of benefit aerodynamically. If you notice, most of the 18 wheeler big rig trucks have a big deflector on top of the tractor cab to cut the air for the top of the trailer... and those guys are all about improving aerodynamics to save fuel. The topper would serve the same purpose aerodynamically in my opinion.


The sticky area would be the distance from the trailing edge of the topper to the front of the camper... if the distance is too great, then too much air can spill into the area between the truck and the camper, and you would see no benefit... the closer the better.


Without wind tunnel data, it would take a LOT of road data collection to definitively say it does or doesn't help and by how much because there are so many variables... but I don't think that it will hurt aerodynamics compared to no topper.
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Old 03-01-2019, 12:40 PM   #4
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Don't forget that a truck topper will add 250-300 pounds to the trucks payload.
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Old 03-01-2019, 01:13 PM   #5
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Don't forget that a truck topper will add 250-300 pounds to the trucks payload.
That's a hell of a topper. I use to take mine on and off by myself. 8' Fiberglass.
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Old 03-01-2019, 01:23 PM   #6
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I'd say that 250-300 lbs is in the ballpark for a good quality topper like Leer, Sportsman, etc. Fiberglass isn't lightweight.


But I think it is a small penalty to pay for all the advantages... especially if you have enough truck and aren't operating on the edge of the envelope.
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Old 03-01-2019, 01:25 PM   #7
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If the topper was built to match the trailer in tow, being as high and very close, might reduce the drag some but enough to gain mileage or fight headwinds? Doubtful.

Might be more effective to add a "bubble cap" to the front of the trailer that helps move the air around that huge "barn door".

If you take a look at Semi's moving down the road today, yes they do have the aerodynamic aids on the cab but they are also adding skirting to the trailers to move air smoothly under the trailer and many even have "tail cones" that can be extended or collapsed as necessary.

Most toppers I've seen aren't high enough to do anything but move the bug splatter line a little higher on the nose of the trailer. I suppose one could have a topper made that would be high enough to help some but then would you want to drive that truck around and not be able to go through a bank or fast food drive-thru because you are now 10' tall instead of just under 7'?
No more parking garages either.

For a while there were a lot of "Midget Racing" air foils being mounted on truck cabs but after people found that they really didn't help all that much I haven't seen one for years.
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Old 03-01-2019, 01:29 PM   #8
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I'd say that 250-300 lbs is in the ballpark for a good quality topper like Leer, Sportsman, etc. Fiberglass isn't lightweight.


But I think it is a small penalty to pay for all the advantages... especially if you have enough truck and aren't operating on the edge of the envelope.
Mine was a Leer. I'd get it off the truck, crawl underneath, lift over my head and walk it to the side of my garage. Balancing was the hard part. Once I found the centers, I marked them inside so I knew where to lift. I'm no body builder...or remotely close, but I don't think my topper was anywhere close to 300 lbs. Maybe today their stronger and heavier, had mine back in the 80's early 90's.
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Old 03-01-2019, 01:30 PM   #9
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You may gain some aerodynamics, but the added weight to your rig will counter any benefit in terms of MPG. Probably for the worst.

I think a better option is to beef up your suspension so that you experience less sway. Think air bags, sway bar, RAS, etc.
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Old 03-01-2019, 01:32 PM   #10
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Whatever you think of the appearance factors and whatever the aerodynamic factors, toppers have some disadvantages.

They decrease payload
They do not work with 5th wheel trailers at all
With most, if not all, security is merely an illusion
Getting to stuff stored up against the cab is difficult and uncomfortable
Down here in the desert, contents can be badly overheated.

For me, having had 2 trucks with paint-to-match caps, the negatives outweigh the positives by a considerable margin
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Old 03-01-2019, 02:06 PM   #11
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My last TV had a very large cap on it, had it on the truck from when I bought it.
I never had any experience towing without it, only with it, and I always thought it helped with reducing the amount of drag hitting the camper.
It sort of worked like a tiered wind breaker, roof of truck / cap / camper.

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Old 03-01-2019, 02:22 PM   #12
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Iíve added a topper on my last 2 trucks. No noticeable difference with mileage or drag, with or without trailer in tow. I just like topper to keep things dry, more secure, and have a place for my dogs.
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Old 03-01-2019, 02:24 PM   #13
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I Installed one one my new Ram and is a Leer. Mine is not close to 300# maybe if like anything else you add options which I did not. Bare bones. Yes I see a improvement on fuel mileage of about 1mpg but I do have a V nose tt which also makes a difference. I can see pulling the tt with flat fronts. I drive at 62 mph when towing so I'm around 91/2 to 10 mpg verses my old truck which got 8 to 9mpg. Later RJD
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Old 03-01-2019, 02:36 PM   #14
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I pulled an 8k lb tt with a cap on my pickup . Got around 8 mpg . Traded in to a 9200 lb fifth wheel. Of course lost the cap and now i get 9.5 mpg . Go figure.
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Old 03-01-2019, 02:36 PM   #15
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I put a cap on my Sierra 2500HD a couple of years in; no discernible difference in fuel economy. However, I do enjoy having all of the enclosed storage space, much like an SUV. I would not go back.

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Old 03-01-2019, 02:48 PM   #16
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I've had caps on my truck before pulling a trailer. I don't currently.

Here is my list of pro's and con's:

Pro's
Gives some security over those that would grab and go something from your truck bed but don't want to risk getting caught busting a window or lock to get what you have stored.
Helps with keeping things dry although not 100% waterproof in most cases.
Appearance if it's something you like (I did, Some don't).
Don't have to tie things down if you are packing it taller than the bed rails and you block and brace everything well.

Con's
Limits you on height of items you can haul in truck bed without removing it.
Not as easy to get into bed and retrieve items as it is without one. Especially a factor as I get older.
No/minimal mileage increase.
Seems to increase the bug splatters on the campers front face if towing at night vs. the bugs being sucked down into the truck bed and hitting the inside of the tailgate without it.
Cost- They're not cheap.


I find it easier to put my tailgate down and get to stuff by either climbing in bed or reaching from the side rails of the truck than I did opening the back or side windows and pulling things through a window. Faster to pack/unpack a truck bed without it than with it. So for me, the negatives of having to tie things down with a couple ratchet straps was far outweighed by ease of loading/unloading. I pack things i'm worried about getting wet in plastic water tight tubs usually (for camping). And I have not had any issues with theft although I try to park where I can see my truck if we are stopping along a route to eat for example. So at this point, I lean against a cap for my personal situation.
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Old 03-01-2019, 03:38 PM   #17
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At first I thought you were asking about those 'wing' looking things that you can strap to the top of your truck to deflect wind over the trailer/5er nose. They are supposed to reduce the smashed bug count on your trailer nose and though they claim increased mileage, my research of the review said most all agreed that any mileage gains were very small if noticeable at all. For me the positives were that they aren't heavy to move around and I think they look cool.

The prime reason I wouldn't and didn't go for one is N.O.I.S.E! Diesel trucks are noisy enough as it is, the number one complaint I read in the reviews was that they add significant cabin noise. The wife and I like quiet time and country music. So no wing for us.
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Old 03-01-2019, 03:43 PM   #18
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A Cap + Airtabs (vortex generators) may show some difference in MPG.
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Old 03-01-2019, 03:57 PM   #19
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I put a cap on my Sierra 2500HD a couple of years in; no discernible difference in fuel economy. However, I do enjoy having all of the enclosed storage space, much like an SUV. I would not go back.



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Thatís the best use of a topper. If you notice that a semi the airfoil is right next to the trailer. Besides the rear of a camper has just as much drag. Plus who drives as many miles as a commercial driver.
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Old 03-01-2019, 04:40 PM   #20
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We put a cab high canopy on our long bed. Seems the weight (premium fiberglass with two full length windows, one which was contractor's style) caused us to lose a mpg or so when towing 30' TT and about half one when not. Unless traveling with the fur kids, it spends more time under cover on the storage rack than on the TV now.
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