Open bed or stock cap which is better mpg-

[Odin]

i did quite a bit of searching here and didn't really come up with the answer i am looking for. my truck is an 84 mazda b2200 diesel that usually gets about 30-35 city and upper 30's highway depending how slow i go, i haven't done any areo mods yet but here is my first opportunity. A stock aluminum cap for my truck popped on craigslist for 150 bucks yesterday but i'm not that interested in a cap unless it increases stock mpg more than just 1%. The cap is only about 125lbs. so the question is will a cap that is flush with the cab increase mpg's enough to make it worth buying?

[wyatt]

Flush with cab and tapered toward a point at the back, yes. Level all the way back, most likely not (likely make things worse). The tapered ones are good for somewhere around 4% better FE.
A tanneau cover is a good option, they improve FE. You can even make one if you want. You could also buy the cap and cut it down to make it an aero cap if you wanted... just remember, smooth transition to no more than 15 degrees.

[Odin]

It will be level the whole way- my three wheeler comes with me pretty much every time i use my truck but it isn't going to fit with a cover only a cap better FE is great but easy use of my truck comes first

[wyatt]

If the cap will take an exposed 4 wheeler out of the air flow (assuming it fits inside), it would be hard to say. If you feel $150 is a good price, and you would have no problem getting your money back out of it, you can get it, test it, and sell it if you aren't happy.

[Odin]

Originally Posted by wyatt

If the cap will take an exposed 4 wheeler out of the air flow (assuming it fits inside), it would be hard to say. If you feel $150 is a good price, and you would have no problem getting your money back out of it, you can get it, test it, and sell it if you aren't happy.

the whole point to this thread was to see if anyone else had experience with this or not- if i wanted to test it myself i would have just bought it and not posted here. my truck is too odd a vehicle i don't think i'll find anyone else looking for my trucks cap

[Frank Lee]

I don't have direct experience but I do recall an old Popular Mechanics article about ecomodding a small pickup and they said a regular decently fitted topper was better than none. IIRC they said it was better than a tonneau too, but I'd have to go find it. Also the LEER topper site had wind tunnel results from toppers and tonneaus.

[Odin]

Originally Posted by Frank Lee

I don't have direct experience but I do recall an old Popular Mechanics article about ecomodding a small pickup and they said a regular decently fitted topper was better than none. IIRC they said it was better than a tonneau too, but I'd have to go find it. Also the LEER topper site had wind tunnel results from toppers and tonneaus.

the issue i am finding is several articles have said it is better but then i've read others that say the drag is way worse

[3-Wheeler]

Originally Posted by Frank Lee

I don't have direct experience but I do recall an old Popular Mechanics article about ecomodding a small pickup and they said a regular decently fitted topper was better than none. IIRC they said it was better than a tonneau too, but I'd have to go find it. Also the LEER topper site had wind tunnel results from toppers and tonneaus.

I would say that the capper would help FE slightly and probably more than 1%

Take a look here:

Some Thoughts on Aerodynamic Shapes

Go to Figure 14.

Notice that the drag of a boxy shaped object gets lower as the length/width ratio is increased. The capper will effectively make your truck longer and this helps your fuel economy.

Jim.

[KamperBob]

I found tonneau beat straight cap for FE but I it really depends on the particular truck and tightness of the nut behind the wheel. The flow field is impacted by many variables including three dimensions (height, width, length) plus ground clearance. Cab shape has more than nothing to do with flow over the bed area as well.

To answer a question with a question, how does your highway FE compare with ATV in the bed versus box empty? That seems like a prudent starting point for your particular situation.

We "might" help with interpretation of field test data but be careful about thinking somebody else's ideal solution will work for your truck just the same. That's a sucker bet.

[Dave's Civic Duty]

Originally Posted by Odin

It will be level the whole way- my three wheeler comes with me pretty much every time i use my truck but it isn't going to fit with a cover only a cap better FE is great but easy use of my truck comes first

Hi Odin,

A 3 or 4 wheeler doesn't fit under the cap of my Dodge 3500. Also the FE would be a wash after the 125 lb weight gain. Mine was a Yamaha 3 wheeler.

Dave