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Old 03-26-2021, 09:13 PM   #15
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Originally Posted by Old-Biscuit View Post

Just look at a map
WY is above CO
That may be a bit of an oversimplification.

It's all downhill to South America?

Thanks, Old-Biscuit, for the humor! You made me smile.

Take care,
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Old 03-26-2021, 09:46 PM   #16
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Wind ?

If you want wind, try driving from Ft. Collins to Casper.
It will either blow you over or feel like you are driving on 4 flat tires.
However coming back from Casper, if you open the doors you'll go 10 miles per hour faster.

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Old 03-26-2021, 10:08 PM   #17
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Originally Posted by Flyer15015 View Post
If you want wind, try driving from Ft. Collins to Casper.
It will either blow you over or feel like you are driving on 4 flat tires.
However coming back from Casper, if you open the doors you'll go 10 miles per hour faster.

Mike in Colorado
Wait 15 minutes and the wind in WY will change directions
Course it will add couple hours to the drive but if you time it just right you can catch a tailwind all the way
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Old 03-26-2021, 10:31 PM   #18
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It takes 4 times the power to double your speed. A 10mph increase at 60 to 70 is a huge fuel blackhole.
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Old 03-26-2021, 10:41 PM   #19
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You’re super lucky to get 15 mpg ever! My 6.7 Dually won’t get that unloaded unless I do 60. I’ve managed to get 13 at 70-72 unloaded before. With my 43’ fifth wheel I average 8mpg and usually do 62-65
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Old 03-27-2021, 07:44 AM   #20
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Originally Posted by HiWay-OurWay View Post
So, hurrying uphill eats up more fuel than driving slower downhill.
Good to know.
Have you never checked your fuel mileage at any speed while going up hill? Coming back from Yellowstone I always get way better mileage on the way back down to TX. Safe travels

Enjoy the journey
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Old 03-27-2021, 08:00 AM   #21
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Originally Posted by bross View Post
I just don't understand why people with RVs are obsessed with mileage, we all get about the same: 7-12mpg pretty consistently. You're dragging your house down the road, if you want good mileage, buy a Prius and stay in hotels.

As mentioned I think it's drag and winds that affect mileage more than anything.
Not obsessed but concerned. When we had our 2500 6.4 gas I got around 8.5-9 mpg. The issue was the 31 gal tank. It was fine for medium length trips but when out in the vast areas of the west I had to be cautious about making sure I made it to the next gas station.
Fast forward and we now have a 3600 CTD that gets 10.5-11.5. Same size fuel tank but now we have an extra 50-60 gals to use between fill ups.

It's just good to know what you limits are.
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Old 03-27-2021, 08:00 AM   #22
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777Driver, Ah yes, a pilot to put physics in nice simple terms just when you need him.

The difference of 1000' over the distance aside, people don't appreciate just how much wind resistance increases with speed. Try manhandling a sheet of plywood around in wind & see how much resistance there is. Our RVs are much more than one sheet of plywood area being dragged down the highway.

One point 777Driver missed is the difference in resistance between sea level & the density of air at Denver's 5,800'. As a sea level dweller I sure notice it. Noticed increased landing speed & take off speed at DIA too.
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Old 03-27-2021, 08:05 AM   #23
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Originally Posted by Jerry Burks View Post
On my way to the RV dealer from Fort Collins, CO to Cheyenne, WY and back in the afternoon with my new F350 6.7 diesel and 35' 5th wheel I experimented with different cruise control settings. Conditions were about the same but maybe 10 degrees F warmer in the afternoon. The 60 miles are mostly flat with a few modest hills but there is about 1000 ft altitude increase to Cheyenne.

The good news: on the way back cruising at constant 60mph on Hwy85 the rig made 15.1 mpg diesel. I think that is pretty decent.

Bad news: on the way up to Cheyenne at constant 70 mph on I25 the truck made miserable 7.6 mpg.

Pavement on Hwy85 is smooth asphalt while I25 is not that smooth concrete. But I still struggle to explain doubling the fuel consumption for only 10 mph difference.

So, just an observation for some idle time discussion
The line that I bolded certainly tells MUCH of the story. That alone would skew your testing, regardless of the speed involved.
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Old 03-27-2021, 08:28 AM   #24
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My ex-Navy father taught me this:

Drag resistance is the square of velocity, and power is the square of energy consumption. Therefore, fuel consumption on flat ground with no wind or grade increases by the 4th power of road speed.

Headwinds and grades only make it worse
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Old 03-27-2021, 10:37 AM   #25
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Me - towing into a 25 mph head wind with a 5th wheel. What is wrong with my truck. It seems to have lost power.

Me -towing with a 25 mph tail wind. Wow, I love this truck and now getting 11.5mpg.
This is easy towing.
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Old 03-27-2021, 12:11 PM   #26
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777Driver clearly shows the obvious part of mpg loss. The OP's mpg difference was about 36% and 777Driver accounted for about 35% loss. Below is a detailed write up on the many forces at play. I know this may be beyond some readers. But at least you can see the complexity and identify some of the factors. Most of these factors are outside of our control once we buy a particular model. The main factors we can control is the vehicle speed, tire inflation and smoothness of the exterior surface. Oh and always drive going downhill!

Vehicle aerodynamics – effects of side winds
1. Vehicle Aerodynamics – Effects of side winds, Vehicle exposed to cross wind

2. Vehicle Body Aerodynamics? Is a branch of physics and is concerned when air flows around a body. Deals with a solid body moving through atmosphere and the interaction which takes place. Depends on varying wind speeds and wind direction. The most important factor is reducing fuel economy in aerodynamics

3. Importance of aerodynamic study?
Drag force is reduced. So maximum speed and acceleration are obtained for the same power output.
Fuel consumption of the vehicle can be reduced to the maximum (about 35% of fuel cost).
Gives better appearance and styling. By reducing the various forces and moments, good stability and safety can be achieved.
Helps to provide proper ventilation system.
Aerodynamic noise could be reduced which results in quiet running of the vehicle.

4. Features incorporated to aerodynamics?
Large corner radii on the front section
Low level front skirts
Fairings above and behind the driver’s cab
Aerodynamically shaped mirrors
Enclosed steps

5. Aerodynamic Drag Aerodynamic drag is usually insignificant at low vehicle speed but the magnitude of air resistance becomes considerable with rising speed.

6. Aerodynamic Drag Drag force depends on the following factors:
The size and shape of the vehicle (area of nose end, vehicle super structures)
Travel speed
Air density
Wind direction and strength

7. FL = ½ Cd Aρv2 FL : Drag Force Cd : Vehicle’s drag co-efficient A : Area of vehicle’s nose end ρ : Density of air v : Velocity of the vehicle PL = FL v = ½ Cd Aρv3 PL = Power to overcome drag

8. Types of Drag
Pressure drag (or) Form drag (or) Profile drag - 57%
Induced drag - 8%
Friction drag - 10%
Inference drag - 15%
Cooling and ventilation system drag – 10%

9. Pressure Drag

10. Typical static pressure coefficient distribution

11. Induced Drag (or) Lift Drag
This lift force depends on the contour of the body
Normal Speed - Not a serious problem
High Speed – Serious problem
Lift affects stability and braking performance
To reduce the accelerated flow upper side
To reduce the deceleration flow under side will reduce the aerodynamics lift

12. Friction Drag (or) surface drag (or) Skin friction
This is caused by friction force between the boundary layer and the body surface
Shear stress generated in the boundary layer
Laminar boundary layer should be maintained
Well polished surface is not only attractive but also makes the vehicle more economical
Body smoothness is of the order of 0.5 to 1.0 microns.

13. Interference drag
The flow over many exterior components interact with the flow over basic body shape and this leads to drag
Exterior components includes door handles, mirrors, aerials and badges which project out from normal surface
Mechanical components Engine parts, suspension system, exhaust system, frame rail
Exterior ornaments must be placed where the velocity is minimum
Door closer must be placed in a close proximity and longitudinally in line with each other

14. Internal Drag (or) Cooling and ventilation system Drag
Arising from cooling of the engine
Brakes, cabin ventilation flows
Contributes 10% of the overall drag

15. Drag Co-efficient The aerodynamic drag coefficient (Cd) is a measure of the effectiveness of a streamline aerodynamic body shape in reducing the air resistance to the forward motion of the vehicle.

16. Aerodynamic forces
Force of air drag in the direction of motion with wind angle along longitudinal axis (Px)
Cross wind force (Py)
Aerodynamic lift (Pz)

17. Longitudinal air drag (Px). The longitudinal component of the resultant of pressure distribution. Magnitude is represented by Px = (Cx p A V2 ) / 2

18. Cross wind air drag (Py). It's formed by asymmetric flow of air around the vehicle body when the wind angle is not equal to zero. Magnitude is represented by Py= (Cy p A V2 ) / 2

19. Aerodynamic lift (Pz). It’s the vertical component of the resultant of the pressure distribution over the vehicle body due to flow of air around it. Magnitude is represented by Pz = (Cz p A V2 ) / 2

20. Effect of cross wind

21. Aerodynamic moments
Rolling moment - Mx
Pitching moment – My
Yawing moment - Mz

22. Rolling moment - Mx. This moment caused by the cross wind force Py about the longitudinal axis. Magnitude is given by Mx = Py a = Cmx p A L V2 / 2. This effect is dangerous for tall van, where side force acts much above the C.G. The only solution is increase the wheel track

23. Pitching moment - My. This moment caused about y-axis by cross wind force Py or the longitudinal force Px. Magnitude is given by My = Pz b = Cmy p A L V2 / 2

24. Yawing moment - Mz. This moment caused about z axis by cross wind force Py. Magnitude is given by Mz = Py c = Cmz p A L V2 / 2

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Old 03-27-2021, 12:17 PM   #27
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The bottom line: drag increases exponentially with speed - not as a straight line.

edit: I didn't read the erudite posts above before posting. I will stay out of discussing intersection/interference drag and compressibility.
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Old 03-27-2021, 01:22 PM   #28
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Ever watch your real time mpg gauge going up hill or even over an overpass? Then down hill?

My gauge will max out mpg going down hill and then do the opposite going up hill.

TFL will get 2.5 mpg diesel towing up that mountain in Colorado on route 70.
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