Diesel Motorhome MPG-- Honest Answers Please

[BillAdams]

We just did a 377 trip from Jacksonville, FL to Stone Mountain, GA.  That's longer than I like but being able to spend an extra night at Stone Mountain vs. Wal-Mart was a great incentive!  That only left us 277 to get from Stone Mountain to Nashville.  We put the bus in the shop Monday morning to get a new full body paint job so we will be in a hotel for about 3 weeks.

Sorry, wandered off topic.  If anyone wants to know more you can PM me or we can start a new topic.

[manholt]

Back to MPG.  If your in a DP and getting more than 7mpg avg. then keep doing whatever it is your doing!

I personally think that most folks, really don't care or pay much attention to it..I did not shell out hundreds of thousand dollars for great mileage!  I did it for the ride and longevity of the Engine, towing ability and larger fuel, water and holding tanks...if I wanted to save $$$ at pump, I would have bought a green car and stayed in Motels!   

[jleamont]

Carl, I agree, you wouldn't get into RV'ing of any sort to save money on fuel to travel. Its all about the journey, destination and comfort ultimately a way of life.

I feel our coach is great on fuel, I have had it as low as 7.2 (I think was the lowest...its been a while) and as high as 10.05, depending on the wind, weight and terrain. I stopped clocking MPG this summer, I told my wife if I have to look at that we shouldn't have bought an RV, she agreed. My MPG went up compared to our last coach, when we stepped up to a DP I told her I was curious as to what it would do but as long as its the same or better I will take it. The only time I would consider rechecking MPG is if it seems to be noticeably worse i.e. something is wrong. I have also learned to drive it differently which seems to be an improvement.

I use to jump in and press economy and step on the accelerator and go. Since I stopped doing that and started to watch my boost gauge, RPM's and now exhaust temp I find myself downshifting it and really has maximized its performance which has improved MPG's enough you can see it.

[five]

17 hours ago, fagnaml said:

... ~2100 RPM and 63 mph which gives me ~ 9 MPG ...

 

Is that RPM correct, that's really high for a diesel?

[wolfe10]

46 minutes ago, FIVE said:

Is that RPM correct, that's really high for a diesel?

I pulled up the Freightliner specs for that chassis.  Cummins ISB 5.9 liter. Shows 300 HP at 2,500 RPM, so cruise of 2,100 would be OK.

You are correct--  too fast for a larger diesel.

[jleamont]

1 hour ago, FIVE said:

Is that RPM correct, that's really high for a diesel?

Five, don't get any ideas, at 2300 yours will throw a fault code at 2600 worry, at 2750 duck . You shouldn't be able to reach that unless the Allison downs shifts during engine braking, if it does achieve that speed step on the brake pedal to get it under control. I am in the middle of a mess now at work with the same engine as you that the transmission over revved during engine braking . To make things worse I have 15 of them all doing the same. Watching the finger pointing is entertaining but aggravating.

This will stir the pot...these tractor trailers are getting 7.4 MPG  Aerodynamics and gearing do a lot

[wildebill308]

On 11/9/2016 at 7:10 PM, BillAdams said:

No worries.  I travel at the speed limit unless the speed limit is above 75.  Between FL and TN the speed limit was 70.  I set the cruise so that I stayed between 68 and 72 MPH (usually closer to 72) and had a great drive at about 5.5 MPG.  300-400 miles per day is my normal day since I travel on business.  Getting there is more important to me than smelling the flowers along the way.

That is not bad for your rig. One of these days you can join the ranks of the unemployed. I once drove from San Antonio to Washington DC non stop. Now 300 miles is a long day.

Bill

[wildebill308]

On 11/9/2016 at 7:03 PM, fagnaml said:

I stumbled on this info exchange after searching for driving speed.  After several trips from Houston to Baton Rouge I found my 2007 Damon Astoria "feels good" at ~2100 RPM and 63 mph which gives me ~ 9 MPG including crosses the big I-10 bridges over the Calcasieu and Mississippi Rivers while towing my Jeep Wrangler.  While I get passed often by all types of vehicles, including motorhomes, fifth wheels and semis, etc. 63 mph just feels right.

And no I'm not a 57 year old fuddy-duddy!

 

Does your touchpad show what gear you are in? Some just show the gear selected. That would be just a little high for my coach running in 5th gear. I have to get to 65 to get my coach to shift into 6th and it drops to about 1650 rpm. So what engine and transmission do you have. I would like to see you bring it slowly up to 66 and watch the tack and see if it shifts one more time.

Bill

[wildebill308]

8 hours ago, wolfe10 said:

I pulled up the Freightliner specs for that chassis.  Cummins ISB 5.9 liter. Shows 300 HP at 2,500 RPM, so cruise of 2,100 would be OK.

You are correct--  too fast for a larger diesel.

I find it interesting that the 5.9 in the Cummins truck has a higher governed speed of 3200 rpm while my 5.9 is governed at 2800 rpm. I wonder if it is just the computer or what exactly is different.

Bill

[jleamont]

10 hours ago, WILDEBILL308 said:

I find it interesting that the 5.9 in the Cummins truck has a higher governed speed of 3200 rpm while my 5.9 is governed at 2800 rpm. I wonder if it is just the computer or what exactly is different.

Bill

Bill, is it the same HP and torque in the truck. Usually an ISB in a truck is not likely to be over 275hp and the torque is lower than the RV engine. I had read somewhere that the head gasket was thinner on the RV for more compression.

[manholt]

WBill, I would guess it has to do with the gears also?

Carl

[augerdogger]

My RV is an '01 and I get from 1.5 to 12 mpg depending on driving conditions.  The RV weighs in from 52 to 54,000 lbs and my towed is a Class B that weighs ~8,000 lbs.  I currently have 133,000 miles on the RV and usually average 6 mpg pulling the towed and closer to 7 mpg without.  My best fuel economy was 12 mpg from Albuquerque to OKC with a 30 - 40 mph tail wind, but payback was the 3.8 mpg driving home into the wind.  The RV has a 500 HP DD with an Allison B500 transmission and I generally run at 1600 rpm.  My previous RV was a '93 with a 500 HP DD and an an Allison 754 that averaged 6+ mpg.  It generally was loaded to 48,000 lbs and it had 190,000 mile when I sold it.  Towing cost about 0.5 mpg.  My first Class A was a single slide that weighed 32,000 with a 300 HP Cummins and a Allison 3000 that averaged high 7's.

 

I will update fuel economy when I have access to ECU.  It stores total fuel used and miles.  My fuel expenses don't discriminate between engine fuel, AquaHot, and 17.5kW generator.

[wildebill308]

I wish fagnaml would come back and talk to us. It still sounds to me he is not geting into 6th gear. 

Bill

[five]

On 11/10/2016 at 1:43 PM, jleamont said:

Five, don't get any ideas, at 2300 yours will throw a fault code at 2600 worry, at 2750 duck . You shouldn't be able to reach that unless the Allison downs shifts during engine braking, if it does achieve that speed step on the brake pedal to get it under control. I am in the middle of a mess now at work with the same engine as you that the transmission over revved during engine braking . To make things worse I have 15 of them all doing the same. Watching the finger pointing is entertaining but aggravating.

This will stir the pot...these tractor trailers are getting 7.4 MPG  Aerodynamics and gearing do a lot

Which transmission do you have, 3000/4000?

[jleamont]

I have the 3000 series in our coach

[five]

Sorry I wasn't more specific, Joe.  I know your MH has a 3000...has to be over 450 HP to have the 4000.  I was referring to your fleet.

[jleamont]

DT-12. It's an automated 12 speed manual. Read up on it pretty neat set up. Put in D and step on the accelerator pedal and it lets the clutch out automatically and shifts by air, outside it sounds like the driver is shifting but they are not. 

[five]

I just binged it...very interesting.  My D/W has a '16 Acura MDX with a nine speed.  You can get dizzy watching the tach...nine is over kill on what is basically a passenger car.

[davidsteverson]

Greetings everyone. I'm a retired CPA so my wife says I have to know all the details - more than I should I'm sure. I keep 2 excel spreadsheets. One is for all expenses related to the motor home. I keep it by category - diesel, repairs, rv parks, and other (taxes, insurance, registration, tags, etc). 

The other spreadsheet I track mileage. First, I post the mileage when we stop for a night. I also track the mileage when we fill up with diesel. The best we have done is 8.2 and the worst is 5.9. On that one I had a 40mph head wind plus I ran the generator for most of the tank. Overall, we are averaging 7.7 on the 17,000 miles we have driven our motorhome since we started on our first trip dec 6 of 2015. 

For what it's worth I tend to drive 62-64 and we tow a Honda CR-V. We tend to drive 500 or so miles a day and then spend 2 - 3 days and then drive another 500 or so miles. That's when we are going from point a to point b. If we are where we want to be we have stayed a month but usually at least a week. We have a 45' diesel with a 450 Cummins. David

[wildebill308]

500 miles a day is more than I want to drive in a day. 250-300 is more my speed. The one thing that fascinates me is how much you can find to see and do in an area, especially if you just stop and look and don't worry about the next stop down the road.

David it sounds like you are geting good mileage. 

Bill

 

[five]

We'd appreciate knowing what coach you have, David, so we'll know the weight and compare apples to apples.

 That 450 Cummins is a bit smaller than many manufacturers would put in a 45' coach.

[fagnaml]

Sorry for not replying sooner.   Career and family have consumed a lot of my time recently.   My 2007 Damon Astoria has the Cummins ISB 300 HP "low torque" engine with the Allison MH-2500 five speed transmission.   The Cummins brochure (attached) for the 2007 ISB Motorhome engine shows torque and power charts over an RPM range of 1600 - 2600 with 2600 RPM being the governed speed.   My cruising RPM of 2100 is "in the middle" of the torque and power charts.

My "refinery operations manager" experience has always shown that operating all types of rotating equipment (pump, compressor, electrical motor, natural gas engines that drive big compressors, etc.) "in the middle of design range" is a very comfortable, very reliable practice.  

My motorhome is my first diesel engine vehicle so I'd like to know from those with years of diesel engine experience 2100 RPM / 63 mph is considered too high for my Cummins ISB engine.  At 2100 RPM the engine feels quite smooth, transmission temperature is ~155 F, engine temperature is in the middle of the gauge and all of which gives me a nice, relaxing ride.

Thanks for your thoughts!

Cummins 300 ISB Engine Brochure.pdf

[wildebill308]

Ok you can't shift into 6th.  I don't think it is to hi of rpm, just higher than what I am use to seeing. I run about 1650 at 65 mph.

Bill

[fagnaml]

Wildebill --

The engineer in me is curious how Freightliner and other motorhome chassis fabricators decide how to "mate" engines and transmissions.   A what engine size would Freightliner decide to use a six speed transmission (e.g. Allison MH-3000) rather than a five speed transmission (e.g. my Allison MH-2500)? 

My Cummins ISB engine is a 5.9L rated at 300 HP which is similar to your Cummins 5.9L (?).  I wish I knew why Freightliner mated my engine with a five speed transmission?  A sixth gear would lower engine RPM and improve mileage at 63 mph.

 

[kingfr]

The decision is probably made by the coach manufacturer rather that the chassis builder. The decision is mostly driven by the price point that they are trying to hit. Allison 2500 is cheaper than Allison 3000, therefore they can shave the price of the coach. You get what you pay for!