RV Drag And Affect On MPG

[RACarvalho]

Originally Posted by The Gritz Carlton

The old GMC...we had one when I was a kid. Beautiful orange and white with 3 tone "shag carpet". Compared to today's floor plans, you couldn't "stand up" in the front. You had to sort of scoot across the engine cover and slide into the seat. It was very low profile up front.

That low profile i what would make it have a better fuel consumption for it would reduce the frontal area BUT, as you notice, would also reduce internal space AND cargo space.
I love the idea of a front drivetrain and front wheel drive but even if someone builds this concept today, it has to be taller and wider because this is what the customer wants and therefore it would defeat the purpose of the compromise-for-less fuel consumption for what Chance started this thread for.

I would drive slower before thinking on downsizing for mpg....

[Chance]

Since I quoted his blog below, I meant to credit his work. I don’t agree with everything he states but makes good sense overall.

https://askthervengineer.com/can-i-i...el-efficiency/

Originally Posted by Chance

.....cut....

“Most RV manufacturers generally DO NOT use wind-tunnel testing to evaluate the aerodynamics of their RV designs. Curb appeal and layout drive the shape, not aerodynamics.”

.....cut.....

For what it’s worth, the reason I started this thread in the first place was to avoid hijacking another where a regular on this forum who drives a Winnebago Class A mentioned getting 8 MPG, and questioned any motorhome except a B-van getting over 10 MPG. Rather than argue, discussing facts in more detail that show why that is not only possible but should be expected was the goal.

A quick search showed his Winnebago Class A is 146 inches tall and 101.5 inches wide. Comparing that to a Phoenix Cruiser at 121 inches tall and 93 inches wide suggest the Class A has “approximately” 30+ percent more frontal area. And since weight is also much higher, one can predict +/- 30% greater fuel consumption — more or less — even if Cd were the same.

So yeah, I would expect a 28-foot E-450 Class C (B+ Cruiser) to get about 30% higher MPG. That would improve 8 MPG to about 10-1/2 MPG. And if we assume the 7.3L V8 is a little more fuel efficient than his V10, and that driver was more conservative due to high fuel costs, then 11-1/2 MPG makes perfect sense.

If one wants a lot more than 11~12 MPG with gasoline using present-day “real-world” technology, it will likely require lower “Area X Cd” or else driving very slowly. It comes down to simple choices we may or may not like having to make or compromise on.

[JimOIB]

Well my 2017 E450 Class C has these nice speed brakes on each side of it that really buggers the aerodynamics and the best I have ever been able to get is 8.8 mpg - typically drive at 58-62mph, at 2100-2300rpm on secondary roads, no tow and lighter loading. I scratch my head when I hear gas Class C getting mpg in the mid-9s and above.

[Bob Denman]

https://www.thorforums.com/forums/at...1&d=1656698096

[Chance]

Originally Posted by JimOIB

Well my 2017 E450 Class C has these nice speed brakes on each side of it that really buggers the aerodynamics and the best I have ever been able to get is 8.8 mpg - typically drive at 58-62mph, at 2100-2300rpm on secondary roads, no tow and lighter loading. I scratch my head when I hear gas Class C getting mpg in the mid-9s and above.

Frontal área and body shape do really matter. Compared to unit I described above, yours is a foot taller and 6 inches wider. Plus it has those “speed brakes” you mentioned. It may be lighter (listed as E-350 on some specs), but a longer and heavier motorhome with better aerodynamics should actually do better in fuel economy on the highway.

We can’t always go by looks, but sometimes differences are hard to ignore.
.

[Chateau_Nomad]

Originally Posted by JimOIB

Well my 2017 E450 Class C has these nice speed brakes on each side of it that really buggers the aerodynamics and the best I have ever been able to get is 8.8 mpg - typically drive at 58-62mph, at 2100-2300rpm on secondary roads, no tow and lighter loading. I scratch my head when I hear gas Class C getting mpg in the mid-9s and above.

Sounds like your and my driving habits are very similar. I take 2 lane highway where feasible, and the speed limits typically are 55... lowered to 35-45 through small towns. This is usually what I would call rolling terrain... hilly, but not steep, with moderate curves. I have driven this type of highway for 50 years... 55 is indeed a safe speed LIMIT.

I have a 2020 (2019 Ford chassis) E-350 class C - 25' bumper to bumper with the legacy V-10. We flat tow a 3,200# TJ about 75% of the time.

I have regularly checked mpg at fillups while traveling. Towing averages 8.5 - 9 in the above conditions... nearly dead on 10 mpg without toad. I have NEVER exceeded 10 mpg that I recall.

That said, freeways with 70mph speed limit, I'm in the right lane with the semis doing 65mph. And the mpg REALLY suffers at 65 on the freeway... typically 8 -9 mpg... WITHOUT the toad.

I'm MY case, the toad adds a 15 - 20% fuel penalty, and the additional 10mph on the freeway adds another 15 - 20% fuel penalty.

[JimOIB]

Originally Posted by Chance

Frontal área and body shape do really matter. Compared to unit I described above, yours is a foot taller and 6 inches wider. Plus it has those “speed brakes” you mentioned. It may be lighter (listed as E-350 on some specs), but a longer and heavier motorhome with better aerodynamics should actually do better in fuel economy on the highway.

We can’t always go by looks, but sometimes differences are hard to ignore.
.

Mine was the first year the model was produced and set on an E450. Thor changed to E350 in 2018. Just FYI.

[Matthew]

Originally Posted by JimOIB

Well my 2017 E450 Class C has these nice speed brakes on each side of it that really buggers the aerodynamics and the best I have ever been able to get is 8.8 mpg - typically drive at 58-62mph, at 2100-2300rpm on secondary roads, no tow and lighter loading. I scratch my head when I hear gas Class C getting mpg in the mid-9s and above.

2020 31y on E450.
9.2 mpg from Charlotte to Key West last week.
Kayaks and electric bikes on back with some drag.
Average right about 10mpg with nothing on back.
68mph.
Also get in the low 20's mpg in Ram 1500 5.7.
I have a contest with employees (delivery drivers) if they can get better mpg than me.
They also drive identical trucks.
I accelerate slow.

[Bob Denman]

https://www.thorforums.com/forums/at...1&d=1656702524

[Chance]

Originally Posted by JimOIB

Mine was the first year the model was produced and set on an E450. Thor changed to E350 in 2018. Just FYI.

Thanks for info. Interesting to me that only real difference between E-350 and E-450 that should affect fuel economy is that all E-450 have 4.56 final gearing, while standard on E-350 (with 7.3L V8) is 4.10.

What’s odd is that when a low-profile aero B+ that is on heavy side, and thus built on E-450 due to weight, is cruising while using less power, it doesn’t get to benefit even more from 4.10 gears. I know that Ford can’t offer too many options due to cost and certifications, but I would be curious to see what MPG a B+ like the Phoenix Cruiser could achieve with taller 4.10 E-350 gearing.

[Lt Keefer]

Thanks for the laughs. If you are worried about fuel economy and it's effect on the family budget, sell the thing and get another hobby. If you are concerned about the environment to a city with mass transit and take the bus to the nearest park. Nothing about an RV is cheap or good for the environment.

[Chance]

Originally Posted by Lt Keefer

Thanks for the laughs. If you are worried about fuel economy and it's effect on the family budget, sell the thing and get another hobby. If you are concerned about the environment to a city with mass transit and take the bus to the nearest park. Nothing about an RV is cheap or good for the environment.

For me it is simply believing that everything can be made better than before, otherwise we would be stuck in the past — not that some things from the past weren’t better, but as a whole we can’t stay frozen in time.

I also think it is normal to put on the brakes as we get older, but I don’t want to accept yet that this is as good as it gets, or should get. I do not know what future motorhomes will be like, but I’m glad we didn’t stay in the past.

One recollection from 60s?
.

[Chance]

Originally Posted by Bob Denman

https://www.thorforums.com/forums/at...1&d=1656698096

Bob, in general I don’t like being negative, preferring to focus on promising opportunities instead, so I wasn’t going to comment. In case of that trailer though, the cost-to-benefit ratio seems completely impractical.

RV compound curves will likely make costs too high and or space less useable. Fiberglass trailers get away with it because they are molded, but even then units like Casita are pricey by comparison and have limited market. Many are not that aero anyway.

What RV trailer manufacturers have had success with is using a sloped or curved roof combined with flat side walls. This must be a good trade off between aerodynamics, costs, and practical use of space.

Sloped roof at back has been common on 5th wheels and is quite beneficial at reducing drag, and more recently arched rooflines have become common on all types of small trailers. If we look at automobiles with lowest coefficient of drag we can see in side profile that arched rooflines are very common. Same concept should work great on RVs at reducing drag without adding too much cost. Designers will have to modify interior floor plan accordingly to make it work though. Some examples below:
.

[Matthew]

Originally Posted by Chance

For me it is simply believing that everything can be made better than before, otherwise we would be stuck in the past — not that some things from the past weren’t better, but as a whole we can’t stay frozen in time.

I also think it is normal to put on the brakes as we get older, but I don’t want to accept yet that this is as good as it gets, or should get. I do not know what future motorhomes will be like, but I’m glad we didn’t stay in the past.

One recollection from 60s?
.

Brings back some memories.
My family had one just like that.
My dad took off three weeks one summer and loaded three kids, a long haired collie, and mom.
We drove from Ohio to Mexico City, with no AC.
No wonder I like camping.

[Matthew]

Originally Posted by Matthew

Brings back some memories.
My family had one just like that.
My dad took off three weeks one summer and loaded three kids, a long haired collie, and mom.
We drove from Ohio to Mexico City, with no AC.
No wonder I like camping.

Sorry, commenting on the classic Winnebago.

[Chance]

Originally Posted by Matthew

Sorry, commenting on the classic Winnebago.

All good.

Talking about Classic Winnebago which had horrendous aerodynamics, in part due to “eyebrow” design at upper front, I noticed Thor just eliminated their eyebrow design on ACE Class As for 2023. It’s probably too small a difference to be measurable during normal driving, but should help a little nonetheless.

[Elite Washington]

There really is only so much you can do. The lower profile vehicles should get better mileage, but if you drive it like you stole it that won’t make a difference. Friends of ours have a 40feet supernova super C. It’s a beast but if they keep the speed around 63 the get a little over 8 mpg. I know we can get above 9 if I keep below 65 in our 32ft class c.

Generally on short trips where time is more valuable then money I don’t worry about the mileage. But on a longer trip I slow it down. I would rather give the money to the small shops/restaurants then to the gas companies and govt taxes.

[plastrd]

Everyone focuses on the front but all the reasons already stated in this thread make that a pointless endeavor because they create compromises.

The other air drag is the big vacuum bubble you're dragging along behind you due to the vertical wall at the back and the lumpy underside and top side of the vehicle. This is where semi trailer manufacturers focus their efforts with side skirting and those trailer tails.

This has the best chance of actually being a cost effective approach where trucking companies have already demonstrated a 1-5% fuel savings (according to wikipedia, plenty of articles also reference numbers in the range). They can be retrofitted to existing trailers/vehicles and there are fully automated ones that open/close based on speed so the user doesn't even need to remember to flip the switch.

That said, 5% of 8mpg is still less than a half mpg gain so you'd really need to put some miles on like those semi drivers to make it a good investment. I'd still like to see some innovations like that in the RV market. Or some innovation anywhere in the RV market if I'm being honest.

[RACarvalho]

Originally Posted by plastrd

That said, 5% of 8mpg is still less than a half mpg gain so you'd really need to put some miles on like those semi drivers to make it a good investment. I'd still like to see some innovations like that in the RV market. Or some innovation anywhere in the RV market if I'm being honest.

As I said, I did add Airtabs to my previous rigs and I did see an improvement in the sway area but not an improvement in mpg.
Airtabs work creating air votex so the "vaccum" created behind the rig is eliminated.
I could certify that noticing that the rear of the rig became clean as the sides after I added the vortex.
Top deflectors would bring an increase in hight what is not desirable and underbelly closure is possible but it will cost some $ due to exhaust insulation needed.

[Cwray]

I’ve tried doing several things like trying using just the overhead AC’s instead of the the chassis AC, etc. and the best results I can get is keeping it below 62 mph with very slow acceleration. That’s really it.

38’ Hurricane (F53) towing a Jeep Wrangler
Driving in the Midwest (so no mountains to speak of)
Letting everyone pass, being patient in slow downs up hill, taking advantage of downhills, etc)

Wifey and I give each other high-fives when we achieve 6.07 mpg. We’re at 12,000+ miles in our fourth year of RV’ing.

She used to be a led foot when she drove, until I made her pump the gas. ;-)

We look at the $0.75 a mile as a trade off to the $60 a night for camping in great parks and making memories with 3 kids and extended family and friends.

If someone could rig those get-ups like you see on semis now that put some fins underneath the body, or collapsible wings at the tail that would cost less than the gains of mpg at the pump—I’d consider it. I don’t see the mag’s sliding the engine out front to put a cone or other aerodynamic front end on it to gain gas mileage. They’d leave the engine and chop off livable area. That wouldn’t work for us.