Tow Vehicle Fuel Consumption - Page 3 - Fiberglass RV


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Old 03-28-2008, 02:47 PM   #29
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My ancient '99 Odyssey has a computer which automatically changes the timing to take advantage of higher octane. Probably in conjunction with a knock sensor (device used by earlier SAABs and others I seem to recall).

Now if I weren't so cheap I would already have tested this out.
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Old 03-28-2008, 08:03 PM   #30
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My truck which i have at present time is a Diesel 6.7liter. On my way to Chilliwack going empty i averaged 20 mpg Imperial. That was a distance of 1164KM. Most of this was highway driving and some town driving. On my way back i took the long hilly way and got 16 mpg Imperial pulling trailer. Trip back was 1429.8 Kms
Lots of my route was in mountains and at times very slow. I never drove over 100kms/60 mph.
Total fuel bill was $483.52 CDN.
Total Kms was 2593.8 Kms
Cost/Km is about $0.18
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Old 03-28-2008, 09:49 PM   #31
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[quote]
Attachment 12305


Nice memories, huh? $5 for 3 Gallons !!
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Old 03-28-2008, 11:00 PM   #32
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That was taken in August 2001, on my way back to Washington from Alaska.

People were complaining loudly about the cost of fuel at that time, they are complaining now and they will be complaining in the future.
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Old 03-29-2008, 02:47 PM   #33
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I just got back from Vancouver Island, towing our new-to-us 14' Surfside trailer. Our 2001 Hyundai Santa Fe (AWD, 2.7 liter V6) got about 24mpg @ 65 mph heading north to Canada, then 17.5 mpg driving back at 60 mph (in bad weather).

This got me thinking about ways I can increase my towing mpg. Here's what I've come up with:

1) You'll notice I slowed down on my way back. Driving slower has a very noticeable effect on gas mileage. With fuel costs soaring, even the big rig trucking companies are slowing down to save fuel.

2) Tires. The trailer came with standard ply-bias car tires inflated to 30 psi. Not only are the tires on the trailer now not designed for towing, they have much higher rolling resistance than a radial tread trailer tire inflated to 50 psi. Replacing the tires should have a measurable effect.

3) Tires on the tow vehicle, too! The Santa Fe tires will need replacing this fall, so i have an opportunity to fix this problem, too.

When you add load to tires they spread out on the ground, increasing rolling resistance without substantially increasing traction. (The weight-bearing part of the tire is at the outer rim of the tire's imprint on the ground; the center section of the tire's imprint bears less weight than the edges do. This is why chronically under-inflated tires wear out at the edges, not in the center of the tread. When the tire's load spread away from the center tread the load moves to the part of the tire designed to handle cornering and turning forces that come at an angle to the tire's direction of rotation. Increasing inflation moves that load back to the center of the tire, which is the part that's designed for traction on the direction of roatation.) The back tires on my Santa Fe have a standard inflation of 30 psi; I could also improve my towing gas mileage and my braking traction by jumping to a tire with a higher load and max inflation pressure rating. Then I could increase my cold tire pressure from 30 to 34 psi.

Tires are rated as temperature range A, B, or C, with "A" being the high rating, "C" at the low end. Tires with "A" ratings generally have harder rubber in the tires and additional structures in the sidewalls that allow them to pass heat up the sidewalls faster to help cool the tires. Both features reduce rolling resistance and improve gas mileage, so replacing my range "C" tires with "B" tires will slightly improve gas mileage, too.

4) Axle maintenance. The last owner never re-packed the axle bearings, so I'm guessing that there's a low-level rolling resistance on the wheels due to poor maintenance. Re-packing the bearings is high on my must-do maintenance list.

5) Air dam. This is something I may have to experiment with: Add a fairing on the top of the Santa Fe that deflects air up and over the trailer the same way that air dams on the roofs tractor-trailer rig cabs deflect air over their trailers. They wouldn't put them up there if they didn't reduce fuel costs.

--Peter
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Old 03-29-2008, 03:56 PM   #34
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If the bearings are a problem they should be heating up and be detectable.

Umm, something wrong with your numbers -- They don't match your conclusion about slowing down, which of course, they should.

From WiKi:

QUOTE
Power

The power required to overcome the aerodynamic drag is given by:

P_d = \mathbf{F}_d \cdot \mathbf{v} = {1 \over 2} \rho v^3 A C_d.

Note that the power needed to push an object through a fluid increases as the cube of the velocity. A car cruising on a highway at 50 mph (80 km/h) may require only 10 horsepower (7.5 kW) to overcome air drag, but that same car at 100 mph (160 km/h) requires 80 hp (60 kW). With a doubling of speed the drag (force) quadruples per the formula. Exerting four times the force over a fixed distance produces four times as much work. At twice the speed the work (resulting in displacement over a fixed distance) is done twice as fast. Since power is the rate of doing work, four times the work done in half the time requires eight times the power.

It should be emphasized here that the drag equation is an approximation, and does not necessarily give a close approximation in every instance. Thus one should be careful when making assumptions using these equations.
END QUOTE

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Old 03-29-2008, 04:02 PM   #35
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OK Pete---Put that in lay mans terms
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Old 03-29-2008, 04:17 PM   #36
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Buried in the middle of that is:

"the power needed to push an object through a fluid increases as the cube of the velocity. A car cruising on a highway at 50 mph (80 km/h) may require only 10 horsepower (7.5 kW) to overcome air drag, but that same car at 100 mph (160 km/h) requires 80 hp (60 kW)."

Double speed, increase power need by eight times!

However, that relationship is less true for lower speeds -- I recall reading somewhere that the air drag factor really starts to come into play at something like 45 mph.

Maybe someone with a consumption readout in their buggy can look at 45 mph compared to 90 mph and give us some real-world readings (without trailer, of course). A lot might depend on the speed at which the car may have been designed to give the least drag.
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Old 03-29-2008, 04:31 PM   #37
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Some comments about tires:

Radial tires have a significant advantage over bias and bias belted tires in terms of rolling resistance. Changing over to radial is a good move from a fuel economy perspective although they tend to be harsher (not a particular issue with the trailer) and more flexible side to side.

Peter's observations about inflation pressure versus tread wear and tread wear patterns are particularly pertinent with regards to bias tires but less so for radials. For radials, decreasing the pressure tends to lengthen the contact patch rather than widen it. However, the decreased bending at both the front and sides of the sidewall work the rubber more and the rubberís hysteresis properties consume more energy and thus fuel.

Rolling resistance versus pressure is not a linear relationship. Once youíre in the design range of the load/inflation pressure table, increasing inflation pressure has a relatively trivia impact on rolling resistance.

Friction is a function of normal load, but again, the friction versus load curve is not straight. Doubling the load does not double the friction. Tread wear generally occurs at the relatively lighter loaded sections. Tread wear occurs at the instant when the tread block just leaves the contact area and it scrapes the road as it snaps from flat contact with the road to itís radial position.

Higher speed ratings are generally obtained by delaying (increasing) the speed at which standing waves occur. Generally this is done in the tread area by altering the resonant frequency of the tread band by adding material such as nylon plies. Standing waves generate heat faster than the tire can shed it and eventually the rubber reverts, adhesion suffers and finally centripetal force does itís thing. Sidewalls are generally kept thin to help them shed heat and itís debilitating consequences.

Lastly, inflation carries the load. The sidewallís just keep the air in. Tires designed for increased inflation pressure generally have stronger sidewalls, are generally heavier and both are bad for rolling resistance.

I know that (some of the rest) of the moderators frown at excessively technical posts but I'm not above tweaking things once and awhile.

FWIW, I have 17 years with a tire company (a technology manager) and for the last 10 years have been a tire engineer for a major auto company.
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Old 03-29-2008, 05:32 PM   #38
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Quote:
Umm, something wrong with your numbers -- They don't match your conclusion about slowing down, which of course, they should.
I read your post three or four times before I realized where you thought my numbers were off, then I realized: You were assuming I drove the trailer both ways, so . . .
Leg 1: I drove the Santa Fe northward without a trailer at 65mph (average) and got 24 mpg
Leg 2: I drove the Santa Fe southward with the trailer at 55-60mph and got 17.5 mpg

The loss in gas mileage was not because I slowed down, it was due to hauling 2000 lbs of trailer behind me.

--Peter
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Old 03-29-2008, 05:38 PM   #39
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Quote:
Some comments about tires:
Thank you Steve . . . You need to teach a class in Trailer Tires for us!
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Old 03-30-2008, 12:22 AM   #40
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Quote:
I read your post three or four times before I realized where you thought my numbers were off, then I realized: You were assuming I drove the trailer both ways, so . . .
Leg 1: I drove the Santa Fe northward without a trailer at 65mph (average) and got 24 mpg
Leg 2: I drove the Santa Fe southward with the trailer at 55-60mph and got 17.5 mpg

The loss in gas mileage was not because I slowed down, it was due to hauling 2000 lbs of trailer behind me.

--Peter
OK! Got it!
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Old 03-30-2008, 01:17 AM   #41
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I know that (some of the rest) of the moderators frown at excessively technical posts but I'm not above tweaking things once and awhile.
I think technical info is a good thing.
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Old 03-30-2008, 01:24 AM   #42
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Quote:
The power required to overcome the aerodynamic drag is given by:

P_d = \mathbf{F}_d \cdot \mathbf{v} = {1 \over 2} \rho v^3 A C_d.
Converted to layman's terms

There are amount of power (energy) needed to push your car through the air is

For 60mph:
It takes 27% more energy to push the air aside when you're driving 65 instead of 60
It takes 59% more energy to push the air aside when you're driving 70 instead of 60
It takes 95% more energy to push the air aside when you're driving 75 instead of 60

And 65:
It takes 25% more energy to push the air aside when you're driving 70 instead of 65
It takes 54% more energy to push the air aside when you're driving 75 instead of 65

So the short rule is: At freeway speeds increasing your speed by 15 miles an hour roughly doubles the amount of gasoline required to overcome wind resistance. You can save a whole mess of gasoline by slowing down five or ten miles an hour.

Another factoid: Most cars get their best gas mileage at speeds between 47 and 54 miles per hour.

So, here's the table comparing efficiencies at speeds over 55mph:
It takes 30% more energy to push the air aside when you're driving 60 instead of 55
It takes 65% more energy to push the air aside when you're driving 65 instead of 55
It takes 100% more energy to push the air aside when you're driving 70 instead of 55
It takes 154% more energy to push the air aside when you're driving 75 instead of 55

And, even more impressive, for 50 mph:
It takes 33% more energy to push the air aside when you're driving 55 instead of 50
It takes 73% more energy to push the air aside when you're driving 60 instead of 50
It takes 220% more energy to push the air aside when you're driving 65 instead of 50
It takes 275% more energy to push the air aside when you're driving 70 instead of 50
It takes 336% more energy to push the air aside when you're driving 75 instead of 50

See the [b]BIG jump at 65 mph? We, as a country, could save a whole mess of gasoline and reduce our greenhouse gas emissions by a [b]HUGE amount by setting our metropolitan area (where the highest number of freeway miles are driven) speed limit to 50 or 55 mph and our rural area speed limits to 60 mph.

Science stuff:

Spelled out mathematically, the amount of energy required to push the air aside is:
1/2 the thickness/pressure of the air
multiplied by your car's windface (the total area of your car or trailer facing into the wind)
multiplied by the amount of aerodynamic drag for your car/trailer (more aerodynamic = smaller number)
multiplied by your car's speed
multiplied by your car's speed again
multiplied by your car's speed a third time

Shortened even more, the amount of energy needed to push your car through the air is
(a bunch of stuff you can't control)
multiplied by your car's speed
multiplied by your car's speed
multiplied by your car's speed

--Peter
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