Diesel Motorhome MPG-- Honest Answers Please

[kaypsmith]

The most reliable source is WW Williams, I also have a dead bus yard, I will PM you with that phone number. Quite a number of parts are also available on ebay, this engine was many produced and used for fire trucks and marine use.  Also if BillA is still looking, the WW Williams in B"ham., Al still has a two stroke mechanic that has help me out with some tough questions, just call them and ask for the two stroke mechanic.

[erniee]

that vintage birds website is just about dead. I'd check with the Prevost Community, which I'm a member of and is quite active- and they know Prevosts

[BillAdams]

The engine was replaced but the problem remains.

[BillAdams]

Most of the folks on the Prevost Community are....never mind......I have not had any luck trying to have a discussion with the folks on that forum.

The POG's (Prevost Owners Group) is even worse!  There is another Prevost owners group frequented only by multi-millionaires looking to save 10% on their camping fees.  I am so far removed from the reality of most Prevost owners that I am officially the red headed step child!

[Cookie]

Even though I don't have a DP, I read this group of posts with interest.  As I read about the MPG numbers, one thing seemed to stand out.  The high speeds at which the coaches were being run.  I know there were the comments that if you can afford a DP, you don't need to look at the fuel expenses.  That may be so for some of you, but some out there can afford the DP because they DO look at fuel costs.  Kind of the attitude that Bill directly above noted in the Prevost owner groups.

My experience with diesels is with 'pullers' not 'pushers'.  We ran 450hp Volvo engines, pulling 60-89,000 (and sometimes 100,000) pounds all around Oregon, Washington and portions of Idaho and Montana.  That also was with 18/26 tires (5-7 axles) on the ground (causing a lot more road resistance than the usual 2-3 axles).  This was connected to a Volvo iShift transmission.  We would average 8.8 miles per gallon based on 3.6 million miles driven per year.  The key to this is that we ran the truck speed limit in what ever state we were driving in.  Thus, 55 in Oregon, 60 in Washington and 65 in Idaho and Montana.  We also ran some in California where the truck speed limit is 55.  Also, the speed limit for ANY VEHICLE TOWING ANYTHING, and of course that includes Motor Homes with a toad, is 55mph.

Even though my Motor Home is only a gasser, it is 57 feet long including my toad, and it drives MUCH more like a truck than my Subaru does.  So I drive it similarly.  Just my two cents worth.

[BillAdams]

We have 5 gears and the newer coaches have 6 gears.  How many gears did you tractor/trailer have?  Also, running between LA and Las Vegas I must maintain 55 MPH since I am towing.  This makes almost no difference in the end result of my miles per gallon.  I would guess that moving from 55 to 70 would reduce mileage by .1-.2 MPG per 10 MPH.  I have never gotten better than 5.5-6.0 no matter where or how I drive the coach.

Just my actual results and that may or may not be worth 2 cents.

[Cookie]

Thanks for the reply Bill.  The iShift had 10 gears.  I am sure I didn't mention that it was an automatic transmission.  The company I worked for went to those a few years ago.

I had always been told that your mileage decreased by 10% for every 10 miles per hour you increased your speed over about 50-55 mph.  Here is a quote I found on a website after reading your post:

"Increasing your highway cruising speed from 55mph (90km/h) to 75mph (120km/h) can raise fuel consumption as much as 20%. You can improve your gas mileage 10 - 15% by driving at 55mph rather than 65mph (104km/h)."

So, if one is getting 6mph, a 10% decrease would be .6 mpg or down to 5.4 mpg. 

My comment about California and speed limits were in response to another poster who said he ran 65 from "Oregon to Arizona and back to Washington, including Tehachaipi and the Grapevine" which means he traveled in California.  Not pointed to you at all.  The reference was just that not all DP owners are the same:  rich and don't have to worry about fuel costs.

Wayne

[CarlAda]

Speaking of speeds, I've seen big rigs doing 80 plus down the interstate -- given tires are probably on rated for 75 mph and the sheer force required to stop these rigs --- I just shake my head and wish them the best as they pass by me.

[Donajoe]

I'm late to this discussion however my advice is that you should not be overly concerned with fuel mileage. If you have a diesel pusher just enjoy the adventure. Make sure you do regular maintenance including all filters and add some injector cleaner every few tanks of fuel. I just finished a 10,000+ mile trip to Alaska, got about 8mpg. But more importantly, the 55 day adventure cost me less than $150 per day, everything included (fuel, food, entertainment, activities). I never worried where I was going to sleep, eat or go to the bathroom. We stopped whenever we found a lake, river, mountain view or valley that we liked. Try doing that in a car that gets 30+ mpg.  Enjoy the adventure, you are driving your personal one bedroom luxury apartment. 

If you don't believe me, figure out what you would pay for gas, lodging, meals and entertainment driving a car on a 55 day, 10,000 mile trip. Joe

[wildebill308]

11 hours ago, Donajoe said:

I'm late to this discussion however my advice is that you should not be overly concerned with fuel mileage. If you have a diesel pusher just enjoy the adventure. Make sure you do regular maintenance including all filters and add some injector cleaner every few tanks of fuel. I just finished a 10,000+ mile trip to Alaska, got about 8mpg. But more importantly, the 55 day adventure cost me less than $150 per day, everything included (fuel, food, entertainment, activities). I never worried where I was going to sleep, eat or go to the bathroom. We stopped whenever we found a lake, river, mountain view or valley that we liked. Try doing that in a car that gets 30+ mpg.  Enjoy the adventure, you are driving your personal one bedroom luxury apartment. 

If you don't believe me, figure out what you would pay for gas, lodging, meals and entertainment driving a car on a 55 day, 10,000 mile trip. Joe

Welcome to the forum. You aren't late just the latest.

Bill

[manholt]

Bill, well said !

joe.  Welcome to the Forum!

I get 6.2-6.8 at 63-65 mph, 1650-1700 rpm on a 425 Cummins, 3000 Allison, 45 foot DP, 54,000+/- pounds!

[Guest]

I was searching for info about fuel mileage and hit this thread.  I've read through all 9 pages.  We are looking for our first motorhome and doing alot of looking and researching.  We want to make a good decision our first time out....good, not perfect.  As someone early on stated, salesmen will tell you anything to sell the unit.  That's why I'm here.  To get the real-world data from every-day users.  I can't contribute RV fuel mileage yet, but I can say my Frontier CrewCab V6 gets 22mpg doing 80 steady.

Until page 8, there were only a few of the 'if you have to ask, you can't afford it' jabs.  Believe it or not, some people do consider mileage, cost or otherwise.  Some are people who are just curious.  Then there are some that look for ways to improve mileage, whether to reduce costs or to extend range for fill-ups.

Anyways, I fit into the curious and the-more-you-know-the-better-off-you-are group.  I do want to know about trends concerning engines, trans, eng/trans combo,  and most of all, the tendencies of a brand/model.  Of course I'm interested in mods that improve performance and efficiency.  Yes, I know there are variables.  And speed, weight, wind, and incline affect mileage.  With all things considered, data shows tendencies.

I doubt that anyone that can buy a new diesel class A needs to worry about the cost of fuel for a vacation trip.  The fuel costs less than plane tickets for a family.  The RV is the price of a decent home in alot of places.  $150k (lower end) is alot of hotel rooms and tickets for the family......like 15+ years of annual $10k family vacations.  I've never taken a $10k vacation.  But I do plan to spend more than that traveling in my RV.

I've driven lots of trucks for duty and lots of U-hauls PCSing.  Getting 4 mpg sucks.  When the same truck only hits 60 downhill, slows to 45 to get over an overpass, and has a 30gal tank it sucks even more.  That same PCS..... stopping every 100 miles for fuel from East TX to CA's Central Coast .....was the worst trip I've ever made.

We all have our reasons for being interested in MPG.  Why knock someone for being curious?  The '....you can't afford it' response is just silly.  You don't know what the other guy can afford unless you are his accountant.  Personally, I want more than 5-6 average.  A gas RV would work for me, but I want the towing ability at speed of a diesel.  I don't want to feel like I'm wringing out the engine to just get over a hill or pass someone.

Again, thanks to those who have answered the OP's question  and provided data for others to consider. 

[wolfe10]

matt2525,

Welcome to the FMCA Forum.

Fuel MPG has increased in both gasoline and post 2007 in diesels as federal standards have tightened.  Yes, on diesels, it comes with the additional cost and complexity of DEF and "converters".

Two other major factors affect MPG:

Speed-- with the aerodynamics of a "house" there is a much larger penalty to pay for higher speed. Many coaches that can get 9-10 MPG at 55-60 are in the 7's at 70.

Weight-- not a significant factor on flat ground, but becomes quite significant on grades.  So, weight's affect on YOUR MPG is dependent on what kind of terrain you cover.

[five]

8 hours ago, wolfe10 said:

matt2525,

Welcome to the FMCA Forum.

Fuel MPG has increased in both gasoline and post 2007 in diesels as federal standards have tightened.  Yes, on diesels, it comes with the additional cost and complexity of DEF and "converters".

Two other major factors affect MPG:

Speed-- with the aerodynamics of a "house" there is a much larger penalty to pay for higher speed. Many coaches that can get 9-10 MPG at 55-60 are in the 7's at 70.

Weight-- not a significant factor on flat ground, but becomes quite significant on grades.  So, weight's affect on YOUR MPG is dependent on what kind of terrain you cover.

....and a headwind, which, IMHO, is the worse detriment to increased MPG.

[wolfe10]

1 hour ago, FIVE said:

....and a headwind, which, IMHO, is the worse detriment to increased MPG.

Yup, Increases aerodynamic drag and with no increase in speed.

Over the years have run into several 25-30 MPH headwinds.  Always when we were on back roads (which we tend to drive anyway), thank goodness. 

So, if we had to drive that day, basically just down arrowed to 5th gear, put RPM at 200 over peak torque RPM and hit cruise.  Ya, in the low 50MPH range, but no worries driving and not terrible effect on MPG.

[BillAdams]

7 to 10 MPG.  There it is!  No one can give you more accurate information.  It depends upon the coach, how it's loaded, how it's driven and the kinds of roads you drive it on.  If that's not good enough you are likely going to just have to give it a try and see what happens.

My coach only gets 5.5 MPG but it's an old technology bus weighting 40.000 #.  As has been said again and again, YMMV!

[wildebill308]

21 hours ago, matt2525 said:

I was searching for info about fuel mileage and hit this thread.  I've read through all 9 pages.  We are looking for our first motorhome and doing alot of looking and researching.  We want to make a good decision our first time out....good, not perfect.  As someone early on stated, salesmen will tell you anything to sell the unit.  That's why I'm here.  To get the real-world data from every-day users.  I can't contribute RV fuel mileage yet, but I can say my Frontier CrewCab V6 gets 22mpg doing 80 steady.

Until page 8, there were only a few of the 'if you have to ask, you can't afford it' jabs.  Believe it or not, some people do consider mileage, cost or otherwise.  Some are people who are just curious.  Then there are some that look for ways to improve mileage, whether to reduce costs or to extend range for fill-ups.

Anyways, I fit into the curious and the-more-you-know-the-better-off-you-are group.  I do want to know about trends concerning engines, trans, eng/trans combo,  and most of all, the tendencies of a brand/model.  Of course I'm interested in mods that improve performance and efficiency.  Yes, I know there are variables.  And speed, weight, wind, and incline affect mileage.  With all things considered, data shows tendencies.

I doubt that anyone that can buy a new diesel class A needs to worry about the cost of fuel for a vacation trip.  The fuel costs less than plane tickets for a family.  The RV is the price of a decent home in alot of places.  $150k (lower end) is alot of hotel rooms and tickets for the family......like 15+ years of annual $10k family vacations.  I've never taken a $10k vacation.  But I do plan to spend more than that traveling in my RV.

I've driven lots of trucks for duty and lots of U-hauls PCSing.  Getting 4 mpg sucks.  When the same truck only hits 60 downhill, slows to 45 to get over an overpass, and has a 30gal tank it sucks even more.  That same PCS..... stopping every 100 miles for fuel from East TX to CA's Central Coast .....was the worst trip I've ever made.

We all have our reasons for being interested in MPG.  Why knock someone for being curious?  The '....you can't afford it' response is just silly.  You don't know what the other guy can afford unless you are his accountant.  Personally, I want more than 5-6 average.  A gas RV would work for me, but I want the towing ability at speed of a diesel.  I don't want to feel like I'm wringing out the engine to just get over a hill or pass someone.

Again, thanks to those who have answered the OP's question  and provided data for others to consider. 

Welcome to the forum. Nothing wrong with being interested in MPG. I pay close attention to my milage. This gives me a baseline so if there is any major change I know something needs attention.  

As it has been said by others, it doesn't matter gas or diesel they get 5-9 MPG. One interesting thing is as engines have evolved they have gotten more fuel efficient. However as this has happened they have gotten bigger and heavier so the milage has stayed about the same. The one exception to this is when they started to use DEF they were able to improve milage. The main reason is they were able to undo some of the changes for emission requirements. One big one is they cut down on how much exhaust gas is re circulated in the EGR system from around 35% to 4-5%.

Bill 

[Guest]

Again, thanks for all the info.  I was figuring around 7-9 if speed and towed weight were kept in check.  You can't expect much more for what amounts to pushing a brick down the road.  I'm mostly leaning towards diesel because I want to tow without much of an impact.  The other advantages are a bonus, though it would be hard to not consider the noise level difference in front vs rear engine config.

If anyone is interested, I have a spreadsheet from when I was pickup shopping.  I wanted to compare gas and diesel ownership costs and the breakover point at which the mpg got me ahead.  Fill in all the costs and it will give you the miles at which the costs are even.  It considers fuel costs, MPG, periodic maintenance, DEF costs, PM frequency, annual mileage, etc.  Just let me know.

[BillAdams]

While the MPG is interesting, how is everyone calculating the cost per mile now that you have to add DEF to the cost per mile.  Is the cost so low as to be negligible?  It's some miles per gallon of fuel plus some miles per gallon of DEF for the real $ per mile.

[kaypsmith]

1 hour ago, BillAdams said:

While the MPG is interesting, how is everyone calculating the cost per mile now that you have to add DEF to the cost per mile.  Is the cost so low as to be negligible?  It's some miles per gallon of fuel plus some miles per gallon of DEF for the real $ per mile.

With a spreadsheet carefully laid out taking all points into consideration? Most companies use P and L statements for same reason.

[jleamont]

If it were me I would factor DEF into cost per mile, most wont as it will make the reality hurt. While DEF would be a start the real eye opener would be maintenance cost per mile on a non emissions coach vs. a newer coach with the same miles. I know what I see at work and the variance is disgusting.

[fagnaml]

I'm like Bill.  My reason for tracking fuel mileage is to have an indicator when the engine isn't performing as it should.   I average 8-9 mpg driving 65 mph on mostly flat roads of south Texas and Louisiana.  My fuel cost "is what it is" as my motorhome is "the" family toy.  The value of my wife, kids, grandkids and me enjoying time together at an LSU football weekend or in the Texas Hill Country is immeasurable compared to the cost of a couple of tanks of diesel.

I am thankful that my motorhome is just old enough that I don't have to mess with DEF (diluted urea). 

[rpelatt]

When we bought our coach it had 19,000 miles on it. For the next 12,000 miles  we averaged 5,2 mpg. while driving 65-68 mph. Then installed the ugli-fix that keeps the EGR from opening and set out on our last trip to the Pacific Northwest. As someone above mentioned the speed limit in Idaho, Washington, Oregon, and Calif. limit Trucks and RVs to mostly 55-60. For that 2700 mile trip we averaged 5.9 mpg. So I realize part of that is from slowing down, but I also felt the Ugli-fix added to our mileage, especially since the final 3 fill-ups yielded 6.2 mpg. As many people know the achilles heel for a Cummins engine is their EGR, so by preventing it from opening I can't imagine that once the entire system cleans itself out that I will stay closer to the 6.0 to 6.3 range. We certainly did not buy a Class A for the mpg's as we feel that sleeping in the same bed, and taking everything we may (or think we may need) is worth it. Just for fun, I usually check local hotel rates near the RV Parks we stay and must say that at the end of each trip after comparing what we paid out vs. staying in Hotels and buying every meal out it is pretty much a wash.

[wildebill308]

rpelatt, Try running Diesel Kleen at about 50% above the listed ratio for a few tanks. 

Bill

[rpelatt]

Thanks Wildebill308. I will try that.