MPG question.

[Shield Arc]

Don't think I've ever posted in this section before, generally stay in the welding section.
Anyway I got a 1,700-mile round trip coming up next week to pick up a tractor. I was wondering if I would get any better mileage if I left the doors open on my dump trailer? With diesel pushing $4.30 a gallon here on the west coast I need all the help I can get! Do you think this would help any, or do I just have wishful thinking.

 

[thatguy]

IIRC - on Mythbusters they tested the pickup truck tailgate down vs up - and the tail cut UP got a slightly better MPG.. It was the swirling action as the air came over the cab that 'pushed' they theorized..

with that said - I have no idea if the same effect would happen with the trailer or not..

A couple other thoughts -

Do the doors have the conspicuity (reflector) tape on them? If so, the doors open may not meet DOT required markings.

IF you blew out a tire it appears that the door latches could take a hit..

Where is your license plate? I dont see it in the 'rear' shot

Brian

 

[Shield Arc]

That is very interesting about Mythbusters! I always leave the tailgate down on my little Toyota pickup.
The doors being open has me worried, looks like a wedge to catch air. Blowing a tire, and having to repair the latch doesn't worry me to much. I know a guy who has a welder, and a torch.

Yes there is reflective tape on the outside of the doors. The license plate is mounted up under the bed, attached to the frame. That is where PJ has the license plate mount and light.

 

[mikehaugen]

Someone told me about that mythbusters episode as well. I did not see it, but the testing was described to me. Let's just say that I was a little... well skeptical (as usual). I have never considered "mythbusters" as a reliable source of information, though it is quite entertaining. I just don't feel that there testing is thorough enough, nor controlled enough to be considered conclusive. There are too many variable to consider to make such broad statements.

That being said, even if you save .5 miles per gallon on a 850 mile trip (I assume that you will need to close the gates on the way back) and let's say that the cummins gets 20 mpg, then the difference between 20 and 19.5 mpg over 850 miles amounts to less than 1.4 gallons of fuel. Is it even worth worrying about and/ or risking the dot non-conformance mentioned above? Not to mention they have got to rattle around a lot more when open I would imagine.

 

[mikehaugen]

Also, the trailer is designed to have the doors shut during transport. The doors are probably designed to give the sides stability when closed, so with them open you are stressing the sides that much more with the wind at 65mph. Maybe not a huge deal but again, is it worth it to save 1.4 gallons?

I have always thought the same about leaving the tailgate down, that thing rattling around and stressing the cables and whatnot is not worth the rather insignificant fuel saving that may or may not be possible. I only leave mine down occasionally for visibility purposes when backing up a small trailer.

 

[sdkubota]

I had my Kubota L3400 shipped 900 miles for $900 through Uship. The trucker who actually hauled the load only got $600. Try and find a direct trucker to haul it back.

 

[Shield Arc]

Someone told me about that mythbusters episode as well. I did not see it, but the testing was described to me. Let's just say that I was a little... well skeptical (as usual). I have never considered "mythbusters" as a reliable source of information, though it is quite entertaining. I just don't feel that there testing is thorough enough, nor controlled enough to be considered conclusive. There are too many variable to consider to make such broad statements.

That being said, even if you save .5 miles per gallon on a 850 mile trip (I assume that you will need to close the gates on the way back) and let's say that the cummins gets 20 mpg, then the difference between 20 and 19.5 mpg over 850 miles amounts to less than 1.4 gallons of fuel. Is it even worth worrying about and/ or risking the dot non-conformance mentioned above? Not to mention they have got to rattle around a lot more when open I would imagine.

Also, the trailer is designed to have the doors shut during transport. The doors are probably designed to give the sides stability when closed, so with them open you are stressing the sides that much more with the wind at 65mph. Maybe not a huge deal but again, is it worth it to save 1.4 gallons?

I have always thought the same about leaving the tailgate down, that thing rattling around and stressing the cables and whatnot is not worth the rather insignificant fuel saving that may or may not be possible. I only leave mine down occasionally for visibility purposes when backing up a small trailer.

All good points, maybe I'm just over thinking this, (insert being cheap here)! That 6.7L Cummins has got up to 23-MPG, but that was at 65-MPH with no load. Not to bad for a truck pushing 500-HP.
2007.5-2009 6.7L Dodge Cummins - H&S Performance

I had my Kubota L3400 shipped 900 miles for $900 through Uship. The trucker who actually hauled the load only got $600. Try and find a direct trucker to haul it back.

Oh this is one of those brother-n-law things, have to pick him up as well. We're gonna build him a log splitter, and install a new loader on his tractor.

 

[mikehaugen]

Oh this is one of those brother-n-law things, have to pick him up as well. We're gonna build him a log splitter, and install a new loader on his tractor.

then isn't he covering the fuel bill?

 

[sdkubota]

If your brother in law is bringing all his belongings...drive away really fast. He is planning on a 45 year "visit".

 

[Shield Arc]

then isn't he covering the fuel bill?

Lets hope so! :laughing:

If your brother in law is bringing all his belongings...drive away really fast. He is planning on a 45 year "visit".

I hope not!

 

[nahfwt]

I actually had an opportunity to do experiments that answer the tailgate up or down question while I was in college. We had a wind tunnel and put a model pickup in it and measured the drag. We tried tailgate up and down as well as off and we had a tonneau cover we put on it. The result was the tonneau cover was the best followed by tailgate up, tailgate off was third and tailgate down was the worst. It turns out that the tailgate traps a pocket of high pressure air in the bed and forces the air to flow around it which acts like a tonneau cover. With the tailgate down it acts like a flat plate and generates much more drag. In another class our professor had a friend who was a GM engineer who came in to talk to us. I asked him about this and he confirmed that they did extensive testing in their full size wind tunnels and said that the trucks are designed to be driven with the tailgates up for maximum fuel economy.

 

[Shield Arc]

I actually had an opportunity to do experiments that answer the tailgate up or down question while I was in college. We had a wind tunnel and put a model pickup in it and measured the drag. We tried tailgate up and down as well as off and we had a tonneau cover we put on it. The result was the tonneau cover was the best followed by tailgate up, tailgate off was third and tailgate down was the worst. It turns out that the tailgate traps a pocket of high pressure air in the bed and forces the air to flow around it which acts like a tonneau cover. With the tailgate down it acts like a flat plate and generates much more drag. In another class our professor had a friend who was a GM engineer who came in to talk to us. I asked him about this and he confirmed that they did extensive testing in their full size wind tunnels and said that the trucks are designed to be driven with the tailgates up for maximum fuel economy.

No kidding!
Live and learn, thank you!:thumbsup:

 

[schmism]

Ive been known to stretch a canvas tarp over mine, even built wooden stays for underneath it to help with air resistance on empty beds

 

[Ductape]

I actually had an opportunity to do experiments that answer the tailgate up or down question while I was in college. We had a wind tunnel and put a model pickup in it and measured the drag. We tried tailgate up and down as well as off and we had a tonneau cover we put on it. The result was the tonneau cover was the best followed by tailgate up, tailgate off was third and tailgate down was the worst. It turns out that the tailgate traps a pocket of high pressure air in the bed and forces the air to flow around it which acts like a tonneau cover. With the tailgate down it acts like a flat plate and generates much more drag. In another class our professor had a friend who was a GM engineer who came in to talk to us. I asked him about this and he confirmed that they did extensive testing in their full size wind tunnels and said that the trucks are designed to be driven with the tailgates up for maximum fuel economy.

I recall reading years ago about GMs extensive wind tunnel testing on pickups. At one time they held a patent on a tonneau cover that only covered the front half of the bed, leaving the rear half uncovered. They found that the air swirling over the top of the cab pushed on the back of the cab, and likewise with the air coming over the tailgate. They found the above mentioned half tonneau cover produced the best MPGs.

 

[meburdick]

Also, the trailer is designed to have the doors shut during transport. The doors are probably designed to give the sides stability when closed, so with them open you are stressing the sides that much more with the wind at 65mph. Maybe not a huge deal but again, is it worth it to save 1.4 gallons?

I have always thought the same about leaving the tailgate down, that thing rattling around and stressing the cables and whatnot is not worth the rather insignificant fuel saving that may or may not be possible. I only leave mine down occasionally for visibility purposes when backing up a small trailer.

This goes for pickup tailgates too.. They're an integral part of the structural integrity of the vehicle.

With regard to the Mythbusters episode... I've read about the principles at work from places other than a television show, and it goes something like this: At slower speeds (say, 30 and under), there isn't enough wind resistance against the gate to be measurable (you aren't getting optimal mileage at that speed anyhow). Once you get to higher speeds, the air can't pass through the seams of the gate as fast as it enters coming up over the roof of the truck. What ends up happening is there is a high pressure area in the bed that the air coming over the cab essentially "bounces off of" and doesn't even end up hitting the gate. It's much like having a tonneau cover on your bed. And, btw, I've always had a tonneau on my trucks, and have NEVER noticed a difference in mileage with the bed covered versus open.

Your 1700 mile trip is two 850 mile trips. Forget about the return since your mileage will be shot anyhow.

How much better would you hope the mileage to get? 5%? 10%? 15%? I'm guessing you get about 18 on on the highway with no trailer. I would expect that to drop to about 15 with an empty trailer. I think the best you could hope for (if those numbers are close) would be to split the difference at 16.5. If you do the math, it comes out to a difference of about 5 gallons of fuel, or just north of $20. To me, it wouldn't be worth it to risk safety or possible DOT fines driving with the doors open.

 

[MN Charlie]

How about doing your own experiment? I would guess that for a 850 mile trip with the trailer empty that you will need to fill the fuel tank at least once, probably half way through. So, leave with a full tank of fuel and for the first leg of the outbound trip leave the trailer tailgate open and calculate the mileage. For the second leg of the outbound trip, close the tailgate and calculate the mileage when you fill for the second time. Ideally, the terrain for each leg of the trip will be similar for the results to be the most relevant and, hopefully, the wind will be similar in direction and speed relative to travel direction.

The pickup bed vortex won't be working in the trailer since it is too far back from the cab of the truck so the truck tailgate up or down studies wouldn't be valid for the trailer so this will be ground breaking research. What a great opportunity to be a science pioneer!

 

[crazyal]

I watched the Mythbusters when they did the tailgate up vs down. They actually did a test with a model, can't remember if it was in a water tank or air. I'm not sure how well it'll translate to a trailer though. With a pickup the cab is higher than the bed, not sure how that would play into it.

 

[Rara Avis]

Oh this is one of those brother-n-law things, have to pick him up as well. We're gonna build him a log splitter, and install a new loader on his tractor.

Have HIM and his tractor shipped via U-Ship by one of those bozos on that Shipping Wars show...it would be both entertaining and cheaper than you have to pull the round trip...

 

[Gary Fowler]

I can tell you that one of those fold up gates with wire mesh on it takes a lot of gas to pull it. I hauled a Goldwing motorcycle for a friend once on a two wheeled trailer with one of the fold up gates. It cut my mileage in half on my old company 6 cylinder truck as it was always downshifting to second gear going up slight inclines compared to when not pulling a trailer. Even with the mesh the air resistance was phenomenal.

 

[sdkubota]

I can tell you that one of those fold up gates with wire mesh on it takes a lot of gas to pull it. I hauled a Goldwing motorcycle for a friend once on a two wheeled trailer with one of the fold up gates. It cut my mileage in half on my old company 6 cylinder truck as it was always downshifting to second gear going up slight inclines compared to when not pulling a trailer. Even with the mesh the air resistance was phenomenal.

The mesh would have the same wind resistance as if it where a solid surface. Anything that disrupts the laminar air surface creates tremendous drag. A radio tower built of trussed steel is evaluated as a solid surface for wind loads. Air does not just flow through the openings.