Camper Shell Impact on MPG

[Gomav]

I went ahead and cut off 4", leaving 4" above the cab. Difference is only 1 on MPG, 41.4 vs. 42.5! What a surprise, and even more so after I added a simple wind faring - 41.9!

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[Gomav]

Look at the top of your truck. The ridges front to back are there to create turbulence ending at the bevel just above the vertical rear window. The turbulence will help reduce the suction of the vertical window at speed. You might try a 60° fairing sticking about 3/4" above top corner to 1.25" below top corner with 3/4" clearance to divert the air to reduce suction. Possibly a short wing with a trailing saw tooth edge ending close to rear vertical of your camper. Maybe only four or five halves of golf balls glued to roof of camper near trailing edge will help. Maybe drill hole through golf balls and just screw them to roof of camper?

Seems to me not so easy to round up all the edges/corners. Also, it does not matter whether the cap is 4" or 8" higher than the cab. Or, the large flat back panel plays the key role here. So now the question is: round corners/ridges or a higher/taller cap, which is more important? The straight way to find out is to cut off 4" more to make the cap and cab flush.

 

[Gomav]

I have the ARE MX. At about 140 lbs did not anticipate much loss. Around town did not notice much change. I do not generally exceed 65 MPH; Only noticed about 1 or 2 MPG loss. My last road trip to AZ high country got ~ 42 MPG round trip. In the past , shells on my 2 Nissan Frontiers with less aerodynamic shells did not have a significant MPG loss.

After cutting off 4", i.e., now 4" higher than the cab like the ARE MX, I constantly get 42 MPG on 55 speed limit highways. The ARE MX looks far more aerodynamic than my bare square box shell, so it seems to me that aerodynamics does not help any. Or, the large flat back panel (door) dominates the drag. With a tonneau hard cover, I get 50 MPG constantly. I'm going to cut another 4" off to make the cap and cab flush, just to see the result.

 

[Raymundo76]

After cutting off 4", i.e., now 4" higher than the cab like the ARE MX, I constantly get 42 MPG on 55 speed limit highways. The ARE MX looks far more aerodynamic than my bare square box shell, so it seems to me that aerodynamics does not help any. With a tonneau hard cover, I get 50 MPG constantly. I'm going to cut another 4" off to make the cap and cab flush, just to see the result.

Good luck, hope it works for you. For me the added head room was important. The 42 MPG that I referenced was a trip from Mesa AZ (elev~ 1350) to Heber AZ (elev 6700) and back; crossing the Rim at elev 7800. Only you can decide what meets your needs. Wind losses at 65 MPH are about 40% more than those at 55 MPH. So if you can keep your speed down MPG losses will be reduced.

 

[MakinDoForNow]

After cutting off 4", i.e., now 4" higher than the cab like the ARE MX, I constantly get 42 MPG on 55 speed limit highways. The ARE MX looks far more aerodynamic than my bare square box shell, so it seems to me that aerodynamics does not help any. Or, the large flat back panel (door) dominates the drag. With a tonneau hard cover, I get 50 MPG constantly. I'm going to cut another 4" off to make the cap and cab flush, just to see the result.

Note the tailgate it has about a 3/4" concave rise on the outside top. This will at speed divert some air vertically and break up the direct air flow by creating some swirling which will help lower the drag formed by the vertical height of the end of the truck. Before you cut off 4 additional inches create something at the top of your cap at the back edge to introduce swirling similar to the top edge of tailgate. To reduce the drag of the large back panel get some air flow onto it. If nothing else cut a 3" hole in the leading edge and several smaller holes in the back panel to let the air reduce the vacuum effect. Or you could put a piece of dryer vent between the holes to just flow air through the cap (Think of driving closely behind another vehicle to get better mpg same principle).

 

[The Real Maverick]

I already have added a simple 45-degree wind fairing. Here is a picture:

Great thread. Great data.

As for the very short shield up front, probably does little to no good. It's way too short / steep to have much effect.

If keeping the height of the shell is important to you, try making a shield that goes all the way to the front windshield.

Best of luck, thanks for posting, I suspect lots of people wonder about this. And keep posting the revisions you make.

 

[Gomav]

Note the tailgate it has about a 3/4" concave rise on the outside top. This will at speed divert some air vertically and break up the direct air flow by creating some swirling which will help lower the drag formed by the vertical height of the end of the truck. Before you cut off 4 additional inches create something at the top of your cap at the back edge to introduce swirling similar to the top edge of tailgate. To reduce the drag of the large back panel get some air flow onto it. If nothing else cut a 3" hole in the leading edge and several smaller holes in the back panel to let the air reduce the vacuum effect. Or you could put a piece of dryer vent between the holes to just flow air through the cap (Think of driving closely behind another vehicle to get better mpg same principle).

There are some vents on the front sides. There are two narrow panels beside the rear large panel. I can drill some holes on these narrow panels, or simply remove them. Would it help?

While driving along the wind, I see lots of vibrations on the back panel. When against wind, no vibration at all.

 

[Gomav]

Great thread. Great data.

As for the very short shield up front, probably does little to no good. It's way too short / steep to have much effect.

If keeping the height of the shell is important to you, try making a shield that goes all the way to the front windshield.

Best of luck, thanks for posting, I suspect lots of people wonder about this. And keep posting the revisions you make.

Right now the height from bed top to the cap top is 20 inches, which is adequate and about right. Before it was 24 inches, I did feel somewhat uncomfortable for too tall. I used 24x48 panels directly, so I did not have to cut anything. If I cut another 4 inches off, then the height would be 16 inches which is really not so adequate.

 

[Gomav]

I removed the two narrow panels on the rear door, virtually no difference, 43.3 vs. 42.5. I then uninstalled the entire rear door, the same again, 42.8 vs. 42.5. Seems the major drag is still from the protruded 4" front.

 

[ThisWas]

I'd say your camper looks pretty good as-is and the mpg losses are relatively minor.

I do appreciate your scientific approach! Now you need to attach short lengths of yarn to all the trailing edges and have a friend video you from another vehicle as you speed down the highway.

Seems to me you're punching a large rectangular hole in the air and there must be considerable drag behind you with or without the rear panel in place.

 

[Deweyordeweynot]

Hello,

I built a camper shell and lost 8.4 on MPG, from 49.8 down to 41.4, running 64 miles at 55 MPH. This is a lot more than I expected, I guess hybrid is far more sensitive on MPG. The shell is only 40 lbs, but 8 inches taller than the cab top. I wonder what if I make it 4 inches shorter?

Before I do the surgery (not reversable), I would like to know how about your shells? The LEER 180 and ARE MX are about 4 inches taller than the cab.

Thanks!

I have the 180XL on order and I anticipate a mileage drop, but it's still going to outpace any vehicle I've ever driven for fuel efficiency. Life is a series of trade-offs, eh?

 

[Deweyordeweynot]

I seem to recall estimating 2-3 mpg loss after installing just the ARE MX, but that was over a year ago. It's heavy, but fairly aerodynamic, IMO. All sides of the MX are angled inward and roof/corners are rounded. I also have clear tape over the entire cap gap so it's seamless. Even with all the highway I'm doing, I'm still right around the EPA 37 mpg combined, truck says 39 but lies by about 2 mpg.

Maybe someone has real and more recent data. Sounds to me like it's more of a theory question. Drag and friction and crap that I don't know about. To me, the mid rise looks proportional to the dimensions of the Maverick.

I have a Leer cap on order and want to fill the gap between it and the cab (I can't believe nobody makes a cap that seals to the cab, like my beloved '85 El Camino had). What kind of tape did you use, and do you have to reapply as time and weather degrade it?
I was considering a foam strip similar to what's used on tonneau covers...

 

[Gomav]

I have the 180XL on order and I anticipate a mileage drop, but it's still going to outpace any vehicle I've ever driven for fuel efficiency. Life is a series of trade-offs, eh?

Yeah, on one hand, 42 MPG with a camping truck is unseen in history, nothing even close; on the other hand, 9 MPG drop from 50 is quite a trade-off. Although my cap weighs only 40 lb and I can easily take it off and put it on by myself within minutes, it is still something I don't want to do. I'll try my best not to trade off with MPG, but make it expandable for more room when needed.

 

[Glen Baker LLC]

I seem to recall estimating 2-3 mpg loss after installing just the ARE MX, but that was over a year ago. It's heavy, but fairly aerodynamic, IMO. All sides of the MX are angled inward and roof/corners are rounded. I also have clear tape over the entire cap gap so it's seamless. Even with all the highway I'm doing, I'm still right around the EPA 37 mpg combined, truck says 39 but lies by about 2 mpg.

There are always trade-offs.
More aerodynamic but heavier.
"There are no free lunches"

 

[Gomav]

I have done more road tests and, suggested by a friend, lots of simulation as well.

I built a very simple 3D model for Maverick in Solidworks and use Flow Simulation to get drag and lift on different configurations, such as bare bed without cover, with tonneau cover, with box cap of different height and tail styles.

The simulation results comply with what I read on the Internet, especially the comments from Chris Coleman:

https://www.quora.com/Do-camper-shells-or-truck-bed-covers-do-more-to-improve-the-gas-mileage

1) The tonneau cover does not affect MPG, maybe even improves a little. Both my road tests and simulation confirmed this, 50 MPG at 55MPH. See the second picture.

2) A box cap increases drag, even at the same height of the cab (16"-17"). See the second picture.

3) If the cap is shaped like the fastback (Cybertruck tail), then the drag can be compensated almost entirely. But a cap of such shape is useless to me. See the 3rd picture.

4) After a 45 degree cut off at rear, see the 1st picture, the cap is still very usable, however unfortunately drag is not reduced enough. See the 4th picture.

5) As long as the cap is higher than the cab, neither 45-degree cut nor fastback can help much. See the 4th picture.

6) There is virtually no difference between a 20" and 24" cap. Both my road tests and simulation confirmed this, MPG drops from 50 to 42. See the 2nd picture.

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