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 > Review on the new 7.3 Ford V*

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ron.dittmer

North-East Illinois

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Posted: 02/04/21 10:29am Link  |  Quote  |  Print  |  Notify Moderator

mleekamp wrote:

I'd say it's more about the wind resistance than the weight of your RV that affects mileage....
I agree that the aerodynamics of the rig will influence the fuel economy. I am no expert, but I feel it could be nearly as influential as the over-all weight.

Some motor homes are flat-faced compared to others built on the same chassis. Then there is the size of the face. Some rigs are taller and wider than others. Given equal weight and identical chassis, a worse-case versus best-case scenario, you would see a change in fuel economy. How much? Who knows. But based on numbers shared over the years, I estimate a 10% difference.

A class C (B+) a best case scenario with aerodynamic cap, angled transition walls, narrow body, and lower roof.
[image]

I wish I could find a better worst case scenario, but here is something.
[image]

* This post was edited 02/04/21 10:43am by ron.dittmer *


2007 Phoenix Cruiser model 2350, with 2006 Jeep Liberty in-tow


DrewE

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Posted: 02/04/21 11:40am Link  |  Quote  |  Print  |  Notify Moderator

ron.dittmer wrote:

mleekamp wrote:

I'd say it's more about the wind resistance than the weight of your RV that affects mileage....
I agree that the aerodynamics of the rig will influence the fuel economy. I am no expert, but I feel it could be nearly as influential as the over-all weight.


Aerodynamics have a much greater effect on fuel mileage than weight for highway driving. On level ground and at a steady speed, the only effect weight has is to (slightly) increase the rolling resistance of the tires. Air resistance, on the other hand, make up the greatest part of the friction that must be overcome, and further it goes up tremendously with speed increases--it's proportional to somewhere around the cube of the vehicle speed, if memory serves.

For climbing hills, and for stop and go driving, the weight is relatively more of a factor than for highway cruising.

The aerodynamics of the rear of the vehicle are at least as important as those of the front. RVs generally are pretty terrible at both ends; a squared off back generates lots of turbulence and drag, which is why airplanes and Priuses tend to have pretty pointy backs.





Gjac

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Posted: 02/06/21 11:19am Link  |  Quote  |  Print  |  Notify Moderator

ron.dittmer wrote:

mleekamp wrote:

I'd say it's more about the wind resistance than the weight of your RV that affects mileage....
I agree that the aerodynamics of the rig will influence the fuel economy. I am no expert, but I feel it could be nearly as influential as the over-all weight.

Some motor homes are flat-faced compared to others built on the same chassis. Then there is the size of the face. Some rigs are taller and wider than others. Given equal weight and identical chassis, a worse-case versus best-case scenario, you would see a change in fuel economy. How much? Who knows. But based on numbers shared over the years, I estimate a 10% difference.

A class C (B+) a best case scenario with aerodynamic cap, angled transition walls, narrow body, and lower roof.
[image]

I wish I could find a better worst case scenario, but here is something.
[image]
How much difference in mpgs or handling between a B+ aero front end and a C with the overhead bed? Does anyone have any data on this? I think it is logical to believe a B+ is better but how much better. The trade off is more room vs better mpg. If only 1 mpg I would choose more space. If the difference was 4mpgs 10 vs 14 mpg I might choose the B+.

BruceMc

Oregon - Willamette Valley

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Posted: 02/06/21 12:05pm Link  |  Quote  |  Print  |  Notify Moderator

Regarding the mileage difference between a C and a B+, I can only offer anecdotal evidence. Our Sunseeker gets 9 1/2 to 10, where my nephews Chevy pickup with the same engine, a 6.0L, always got 12. And that's just a pickup.
As far as the Ford engines, it's probably a lot of the same story. The V10 is a gas guzzler, no matter what, and I wouldn't expect much different out of the new Ford V-8.
But oh the power difference…


'16 Forest River Sunseeker 2250SLEC Chevrolet 6.0L
Previously:
'00 Four Winds 26Q Class C (Ford E350 V10)
'96 Kit Sportsmaster 212f Fifth Wheel/'93 GMC Sierra K2500
'91 SunLite poptop truck camper
and the first: a Wildernest flip-top canopy.


JaxDad

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Posted: 02/07/21 06:47am Link  |  Quote  |  Print  |  Notify Moderator

I think you’ll find the issue of fuel mileage is tied far more to the drivers right foot than it is the aerodynamics.

The ‘sweet spot’ of the drivetrain is also important. For a few years my father & I both had exactly the same make / model / drivetrain Class A’s. He swore up & down that driving slower, 60 mph gave him the best mileage. I disagreed. Then we went to the same place at the same time. He held 60, I drove 65 - 70. Low & behold my rig used less gas to do the trip.

ron.dittmer

North-East Illinois

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Posted: 02/07/21 07:16am Link  |  Quote  |  Print  |  Notify Moderator

Gjac wrote:

How much difference in mpgs or handling between a B+ aero front end and a C with the overhead bed? Does anyone have any data on this? I think it is logical to believe a B+ is better but how much better. The trade off is more room vs better mpg. If only 1 mpg I would choose more space. If the difference was 4mpgs 10 vs 14 mpg I might choose the B+.
That is a very good question. I have no supporting data, only my gut feeling based on reading years of comments from other people regarding their own fuel economy. Comparing our aerodynamic rig to a typical class C of the same length with bad aerodynamics, the difference is not more than 20%. But there are just too many factors to state a reliable figure. Like others mentioned, aerodynamics are more critical if you cruise at 70 mph versus 60 mph. Some people live close to terrain. We drive across the Great plains under better mpg conditions, factors like that.

There are unrelated benefits to having a seamless B+ cap, primarily avoiding the resonation of the over-hang which encourages seams to leak on a class-C as it ages.

bobndot

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Posted: 02/07/21 08:44am Link  |  Quote  |  Print  |  Notify Moderator

I agree Jax, Gjac, Bruce, Ron.. I would buy the floorplan that works for you , its about comfort bc this is our fun time hobby.
I would miss the overhead bed in case we need it as well as the addition storage. Look at the way we spend out time each day. 1/3 is sleeping, so a comfy place to sleep is important. 10 mins each day in a shower is a little less important. I can get by with a little smaller shower to have a full 60x80 QB.

I agree Ron, about 10% difference is a good ballpark difference. I like the aerodynamics of the PC and other rvs like it for crosswind/headwind handling reasons but not for the mpg difference.

It reminds me of all the AirTab vortex rv and OTR trucker threads that say it's a minimal mpg difference. Even if it was as much of a 10% difference for rv use where most of drive 5000 mi each year, It's not worth thinking about it. Its one 55 gal tank of fuel difference each year to run 5000 rv miles.
For OTR trucking companies running a millions of miles on a fleet of trucks is worth thinking about aerodynamics , its a business not a hobby.

Gjac

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Posted: 02/07/21 10:52am Link  |  Quote  |  Print  |  Notify Moderator

Ferrari said "Aerodynamics are for people who can't build engines". I think an aero front would have a greater impact on a car going 180 mph vs a MH going 60 mph. Kind of like ram air. Every thing is a trade off in engineering terms. I like the lower profile of the B+ and maybe lower CG, but for 1 gal more mpg I would take more storage if everything else was equal.

Gjac

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Posted: 02/07/21 11:07am Link  |  Quote  |  Print  |  Notify Moderator

JaxDad wrote:

I think you’ll find the issue of fuel mileage is tied far more to the drivers right foot than it is the aerodynamics.

The ‘sweet spot’ of the drivetrain is also important. For a few years my father & I both had exactly the same make / model / drivetrain Class A’s. He swore up & down that driving slower, 60 mph gave him the best mileage. I disagreed. Then we went to the same place at the same time. He held 60, I drove 65 - 70. Low & behold my rig used less gas to do the trip.
I came to the same conclusion after 100k miles of driving and engine mods. Headers, FF mufflers, CAI, advancing timing improved performance by 30 % or so, but no increase of MPG. When I added a vacuum gauge I had in my garage for 40 years and used it my mpgs went up about 1/2 mpg if I use it. The best mpg was above 10" HG. I could also tell when transmission was about to shift from the gauge so I would just back of the throttle to keep it in 4th gear.

JaxDad

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Posted: 02/08/21 05:50am Link  |  Quote  |  Print  |  Notify Moderator

bobndot wrote:

I agree Jax, Gjac, Bruce, Ron.. I would buy the floorplan that works for you , its about comfort bc this is our fun time hobby.

I would miss the overhead bed in case we need it as well as the addition storage. Look at the way we spend out time each day. 1/3 is sleeping, so a comfy place to sleep is important. 10 mins each day in a shower is a little less important. I can get by with a little smaller shower to have a full 60x80 QB.


You bring up a good point I’d forgotten about earlier. I use my C very differently than most, I express to my destination (year round) where I stay in it for business. Typically I boondock enroute and find that sleeping over the cab is the warmest (over the engine and with interior space below me) and definitely quieter than sleeping on top of the genset.

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