Open bed or stock cap which is better mpg-
 

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?

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.

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

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

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 :rolleyes:

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

Take a look here:

http://ecomodder.com/forum/showthrea...pes-11183.html

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.

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.

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:turtle:

i've already measured and it will fit inside, my three wheeler is long and low

mid 30's WITH the three wheeler in the back, high 30's without it as there is no room in my garage i just keep it locked in the back of my truck most of the time and really don't notice that big a difference but i drive very slow 55-60 at all times on the highway hugging the right lane and 95% of my road time is highway - i think i'll take "3-wheelers" advice and try out the cap and report my findings-
thanks for the help everyone!

Many years ago, I had a F250 with a cap that was higher in the middle by a few inches. After I took it off (not that I tracked it very closely back then, but) my mileage seemed to drop pretty significantly.

As another benefit to having it on, I could park in the Texas sun, with my back to the west in the afternoon, and the cab would stay a lot cooler. Also, the way the air would flow, if I opened the rear window of the cab, but closed the pass-thru window of the cap, I could get a fairly pleasant breeze, similar to having a sunroof.

For only $125, I would think it's certainly worth a try. The benefits extend to beyond just FE.

92 toyota did better with it
 

A friend has a 1992 Toyota pickup with cab profile all aluminum cap.Vehicle logbook shows around 3-mpg increase over open bed, 28 vs 25mpg.
The T-100 I have at one time had a Leer straight topper.Memory tells me that it showed better with than without.I'll check.
What your looking at,is a 2-door sedan without a trunklid compared to a station wagon.
Intuition would tell me that it would be impossible to raise the drag with a cap as long as it does not increase frontal area.
Old 1960s SAE papers suggest that an open pickup and suburban type vehicle are equal.If that were true today then it would be a wash and I'd hate to try and live on the difference.

All three pickups (a Toyota SR5 and two Ford Rangers) I've owned got better fuel economy with a standard topper on than with just an open bed, typically about 10 % better.

I've never tried a tonneau.

With my present truck, I did try it with an open bed and the tailgate on and off (not down, completely off). I didn't notice any difference in FE, though I probably wouldn't have noticed anything less than a couple of percent difference.

I bought the cap, so as soon as i run a few tanks with my bed empty uncoverd i'll bolt on the cap and post up my results!

took a bit but i got some results, unfortunately i am terrible at only doing one thing and testing it so i might have botched the results- I got the cap put it on, but i also changed the oil, changed the air filter, changed the fuel filter, got new tires, lowered the truck 4 inches, had the tires aligned and the temp went from mid 20s low 30's to the 50's- I've run two tanks through it so far, first tank measured 44mpg second tank was 45mpg at 90% highway driving - my highest to date i remember was 39 so its an improvement for sure... I think next tank will be without the cap again as to see how far it drops.
What i can't figure out is how the hell they came up with the 23city 29highway EPA rating I've never gotten anywhere near that even with almost 100% city its in the low 30's-

Frank, I agree 100%.

Anytime one makes the length/width ratio of a body shape longer, the air drag is decreased. Think of a streamlined train.

Take a look at the link below, and specifically at Figure 13 and Figure 14.

http://ecomodder.com/forum/showthrea...pes-11183.html

Also tried looking in the Stickies for Phil Knox's articles on truck air drag but did not find what used to be there.

Jim.

Cap versus tonneau really depends on geometry. If the box is so short that flow cannot reattach to a tonneau then a cap should show better MPG (cleaner wake). If the box is long enough that flow does reattach to a tonneau then a cap should show worse MPG (larger wake). There are too many dimensional variations between makes and models of pickups for one-size-fits-all rules to apply. Phil's template is an excellent tool for visual prediction of whether a given tailgate is reattachable or not.

Exactly. Tonneau covers do nothing at all for Dodge Dakota quad cabs. I have fuel consumption data that proves this.

I always figured that the cap would yield better mpg too. However, at work we put caps on about 50ish 2001-2005 Ford Rangers. My buddy at work is in charge of paying fuel expenses for our fleet. He said that mpg has decreased accross the board about 1.5mpg since the switch. We still have 40 to 50 trucks with open boxes for comparison.

I think alot of it has to do with how long the box is and the shape of the cab (as mentioned above). Aparently the regular cab Rangers w/ 6 foot boxes do not benefit from the extra weight/drag of the caps. I'm sure this would be a different story on a 1/2 ton truck with and 8' bed though.

More info on my trucks: All are/were extended cabs with 6 foot beds and 5 speed manual transmissions. And, as I said in my earlier post, all three got better fuel economy with the toppers on than with open beds.

It would be nice to hear more experiences from other drivers and compile these experiences in a table and see if we can deduce any trends from that, rather than just guessing and theorizing, though the guesses and theories are interesting, also.