Fuel Mileage Puzzle

I have been following a thread on my travel trailer owners forum; the thread chronicles the RV trailers a guy is delivering for a living. He is in Canada, and uses a 03 3500 DRW with CTD 5.9. He has over 200,000 miles of doing this listed so far.

A number of folks have asked about the economics about doing this for a living, and he has often posted the fuel mileage he gets with the different trailers. On a recent post someone asked again and he noted that: (1) fuel mileage is worst with park model trailers, (2) travel trailers do a little better, and (3) fifth wheels "always get 2 - 3 mpg better, regardless of size or weight."

The comment on fifth wheel trailer surprised me. I have learned here on the forum that shape of the trailer is as large an influence as weight - but a lot of fifth wheel trailers are quite tall and heavy. What else is going on?

Thanks!

 

They are closer to the trucks cab. Most now of days have nice front caps without square corners, and many have rear caps with round corners. Chris

 

I'm sceptical of the 2-3 mpg advantage while pulling a 5vr, and the term sceptical is being kind. 75-80% of the miles on my 03 have been towing, mostly my 30ft toybox 5vr, but I also have a flat bed and a 7'x14' enclosed. My 5vr is definitely the heaviest(8000k unloaded/14000k loaded), and is apples to oranges as far as my other two trailers, the mileage I attain pulling the 5vr ranges from 9-12 mpg, the other two trailers depending on load and weight is 12-15 mpg.

If a comparison between a bumper pull and a 5vr of equal weight were done, I would be totally surprised if the same towing vehicle(the 03 mentioned) pulling the 5vr were to deliver a 2-3 mpg gain.

 

The 5er rides in the trucks airflow, reducing drag. A TT, especially if you have a long shank, gets too far from the truck to stay in the "disturbed" air.

A friend did a bunch of different towing with his 02 Auto. A 35' 5er with a motorcycle trailer tucked in behind the 5er netted the same mileage as a 18' TT at about 1/3 the weight and less total height.

 

OK, so let me see if I am understanding this. For simplicity, let's say we have the same length and weight trailers, but one is a bumper pull and one is a fifth wheel.

I picture the truck pushing air out of its way - to the side and over the top of the cab.

In the case of the fifth wheel, the air continues to be pushed up and over the top of the fifth wheel trailer, where it flows along the top of the fifth wheel trailer and then back down at end of the fifth wheel trailer.

In the case of the bumper pull, the air falls down to the bed of the truck, down off the back of the tailgate, and then strikes the front of the travel trailer. At this point, the air is pushed off to the side and up again, as it was when the truck first encountered the air.

That is, it is this pushing the air to the side and up twice for a bumper pull rather than once for a fifth wheel that allows a fifth wheel better towing efficiency.



Is this the general idea?



Also, as some have pointed out, I would guess that the more the front of your trailer is shaped like a bullet the more efficient your towing will be, regardless of whether fifth wheel or bumper pull. Does that make sense?



Is this also why I have read that sway is much less of an issue, or not an issue, for fifth wheel trailers versus bumper pull trailers?



Thanks.

 

You have the air flow right. The reason the 5th wheel tows better might have a little to do with air flow, but the major issue is that the trailer attachment point is at the center of the rear axle or slightly forward of the point, so side to side movement has no lever to effect the TV like a ball hitch that is 3-5 feet behind the center of the rear axle. Chris

 

I spent ten years transporting trailers to dealers.
I agree 100% that the worst thing to pull is a park model. One only has to look at the front of these things to see why.
I am not so sure about there being that big a difference between the fifth wheel and the bumper pulls on mileage but my records aren't all that inclusive.

There is a big difference between the average box trailer and the Airstream style of trailer, That one will be in the 2-3 mpg area. I would notice little difference in mileage between a 16 ft and 34 ft Airstream so the aeorodynamics are a much bigger factor than the weight.

Anything with a flat high upright front , like an extended height cargo trailer will pull like a rock regardless of weight

 

How does the side to side movement effect mileage on the 2-3 mpg level? Or are you talking about other differences?

 

RUSSELL5000 wrote:

"Is this also why I have read that sway is much less of an issue, or not an issue, for fifth wheel trailers versus bumper pull trailers?"

I was answering that part of his question in post #5 regarding 5th wheel stability. Chris

 

That makes sense. Thanks.

 

Try to imagine holding a 4x8 sheet of plywood crosswise to a 60 MPH wind. That's about the frontal area of an RV above the tow vehicle. Wonder why fuel economy drops off towing?

 

does a air wing on top of truck help?

 

I haven't towed a fifth wheel trailer, but I imagine some are better than others in frontal area and hence mpg of the truck. I can say that towing at 72-75 mph I get around 1.5 mpg better with my 21' Airstream that is 8' wide at about 4500-5000 lb than with my 8.5 ft.wide box trailer. The latter can be empty at 3800 lb. or loaded at 7500-8000 lb, getting about the same mpg either way. I was a bit surprised that the Ram got noticeably worse mpg with it compared to the Airstream or an open car trailer, and more so that mpg was about the same empty or loaded.

 

Thanks to all; I am just kind of surprised that the impact due to shape of the trailer is as great as it is. Dan's analogy is a good one though, and I have held a 4x8 sheet when a light breeze took it and it is amazing how much force it exerts. This is confirms again the benefit of the Airstream trailer shape.

 

I'm not skeptical at all. The reason your 5er gets poorer mileage is that big, flat, squared off rear door. You might as well be pulling a parachute. If it is aluminum skin (the kind that looks like corrogated siding) that just makes it worse.

I have pulled TTs and 5ers (not toy haulers) of the same weight and definitely see better mileage with the 5er. The absolute worst is an aluminum skin toy hauler TT. Even if it weighs 5000 pounds empty it will give me worse mileage than a 11,000 pound 36 ft (now defunct) Alpenlite.

 

I believe my mileage with my 5th wheel is quite normal, comparing it to other similar rigs. I also expect to get less mileage pulling it vs my other trailers, as I stated, they are apples to oranges, not comparable. What I am "sceptical" about, is the 2-3 mpg difference, between a bumper pull and a 5th wheel of equal size and weight. Having pulled dry vans, refers, and flat beds, I acknowledge the wind resistance makes a difference, but 2-3 mpg is huge, and I'm just not sure I buy it...

 

No. It won't be close enough to the trailer to actually divert air over the trailer, in fact will create addional drag by being a parasite. If it was close enough to the trailer it is not large enough to deflect air from all the frontal area of the trailer. If it were wide enough to cover the entire front, the downforce would cave in the roof of the truck, or if bed mounted would add the equivalent of hundreds of pound to the load on the suspension. In addition, your pickup isn't as wide as the lower portion of the trailer either, and is contoured, being narrower in the front and rear than in the center. That dynamic diverts air flow to the front of the trailer anyway.

 

You could always do what I do regularly. Hook up 'em up and see for your self. BTW, your toyhauler will produce, at the very least, 1 mpg lower mileage than a regular 5er of the same size and weight.

 

I have an 18 foot, 900 pound utility trailer with steel mesh ramps on the rear. Unloaded, I consistently got 12.5 mpg. With my 3000 pound car loaded, I got 15 mpg. With my 4000 pound Jeep, I got 16. The taller the car, the more wind was diverted over the ramps and the better fuel economy I got. Apparently weight has no effect on my truck.

 

The square back end of a toy hauler is a HUGE drag. A nice aerodynamic 5th wheel with rounded corners and smooth sides could absolutely produce 2-3 mpg better than a non aerodynamic travel trailer. I used to own a 10'6" cab over camper and a 38' triple slide 5th wheel at the same time. Driving the same roads going to the same places sometimes with the 5th and sometimes with the slide in, the 5th wheel was ALWAYS 2mpg better. I'm certain it's because of the flat nose on the camper that was overhanging the windshield. Created a huge pocket to trap air. And the rear was pretty square which greater drag.

Best thing I ever did to my current toy hauler and my bumper pull enclosed trailers was add a set of air tabs to the back. About .5 - 1mpg better mileage, but much more stable in side winds and the rear of the trailers stay considerably cleaner.