Diesel Motorhome MPG-- Honest Answers Please

[five]

We concur. We have a 2006 Itasca Horizon with a 400 Cummins and tow a 2011 CRV. Its always the same @ 6.5 to 6.7 mpg. Full timers, but we've only gone 14K miles so far. The east coast is OK but were looking forward to getting back west of the Rockies.

How fast do you cruise??? My Allegiance, with a 400 ISL, GW 34k, pulling a 2014 CRV at 60, averaged a bit over 8 mpg. Once you go over 60, mpg goes down. My mpg never went under low 7s...it went that low once when bucking a head wind so strong it pushed one of my roof mounted wind shield wipers to the straight out position.

[jleamont]

How fast do you cruise??? My Allegiance, with a 400 ISL, GW 34k, pulling a 2014 CRV at 60, averaged a bit over 8 mpg. Once you go over 60, mpg goes down. My mpg never went under low 7s...it went that low once when bucking a head wind so strong it pushed one of my roof mounted wind shield wipers to the straight out position.

I was thinking the same...... So far the worst we have seen has been 8.5 best 10.1, (generator on and towing) wind, speed and terrain really dictate the mpg. I was told the ISL with the VG Turbo (2006 should be equipped)was better on fuel than my 01.

I am running all synthetic lubes in the drivetrain and I have low rolling resistance tires, I am sure that might account for part of the reason for the decent mpg.

[mshudy]

We have a 2015 Holiday Rambler Ambassador, 33,000 pounds and we tow a 2013 Ford Explorer, about 4,400 pounds

We have been to many places with hills and mountains.

My overall average MPG is 10.2 MPG with a Cummins 340 HP engine

[five]

As a suggestion, in addition to mentioning the horse power, tell us which Cummins it is (ISB, ISC, ISL, etc) would help. More than one engine puts out the same HP.

[w5txxt5w]

Well here is my info. 2001 Bounder 37K 275hp ISB 140K miles. Running weight average about 24,500 lbs. Over the period from 2001 to present have towed, 1998 Wrangler, 2004 Liberty and currently a 2011 Honda CRV (2wd). Mostly dry camping, so fair amount of generator usage included (1200 hrs) and mpg average over the entire period is 9.1mpg. Travel mostly in Western half of US and 3 trips to AK thus substantial amount of mountain driving. Tow generally around 60mph and use both economy and performance mode of Allison MH3000 and lots of cruise utilization. Have disconnected the Banks as no benefit observed other than slight performance (acceleration) from stop. No mileage improvement observed and have had some reliability issues.

[Frankturman]

My feeling is if you have to ask you can't afford the coach!

[augerdogger]

What make model coach do you have? Several of us here are trying to find other owners with the 500hp DD.

I have a '01 Featherlite DS conversion. The shell is a 2000 Prevost H3-45 VIP. I wanted a coach with a 500hp DDEC III but settled for 500hp DDEC IV. The fuel economy and performance difference between III and IV is huge. I settled because we only drive 10-15,000 miles/year. Previous coach was a 500hp DDEC II that we drove 15-20,000 miles/year. AH uses both 0.35 and 0.40 gal/hr nozzles. I have used both and currently have a 0.35 installed. IMO I had more carbon buildup with the 0.40's.

[marvnoren]

I have a new Solie 38 and without towing and in my "sweet spot " of 63 mph I get 12 mpg.

[sergeantsue]

I have a new Solie 38 and without towing and in my "sweet spot " of 63 mph I get 12 mpg.

Is that the 340 Cummins?

[marvnoren]

Sorry, yes it is with 2500 Allison transmission

[RABEMI]

'04 43' Mountain Aire with 400 ISL Cummins, towing '08 Silverado 4X4. Mpg Avg. 6.8-7.3 at 60-70 MPH. Nearly everyone I talk to about this, with comparable rigs says about the same.

TP Texas

[rhart]

In 5 or 6 mos. I plan to buy a 2000 to 2004 or so bus. Newmar, Country Coach, Monaco type class A. I've read with interest this whole thread. In addition to trying to understand the difference in these buses I need to learn the different type diesel engines. Is there a place you long timers can refer me to in order to learn things like what DD means? I know what DP means but I get lost with all the other abbreviations.

Also I suppose it would be impossible to find out "bad years" or "bad engines"to stay away from??

Thank you

[wolfe10]

DD is Detroit Diesel.

What other abbreviations would you like "translated"?

[rhart]

Hi Brett

DDEC

DSSP

ISL

ISB

Since I'm starting with a clean slate is there an engine from which it is wise to stay away? I'm told diesels should go 1,000,000 miles but I'm sure they must need to be rebuilt every so many miles. Is there a rule of thumb? 200,000 to 300,000 miles? I notice coaches with 60,000 miles on them are way more expensive than 120,000 mile rigs by many 10's of thousands of dollars. Suggesting it is very expensive to rebuild a diesel engine.

Thanks for your input

Rick

[wolfe10]

DDEC: Detroit Diesel Electronic Control (DDEC) system.

ISL: Cummins 8.9 liter engine. 370-400 HP. Most 1,200 lb-ft torque. Can have either engine compression brake or exhaust brake.

ISB: Cummins engine. Early ones 5.9 liter. Later ones 6.7 liter. Cummins smallest engine used in motorhomes

Properly cared for, the diesel engine will outlast the coach.

[rhart]

Is there one or the other that is considered to be an engine to stay away from? Or are you trepidatious about answering because one of your friends has one. Any person you may insult probably will not see your answer. Or perhaps this is a good question to answer via PM.

Also is compression brake better or not as good as exhaust? A "jake brake" is which type?

I appreciate your answers to these newbie questions.

[wolfe10]

I know of no bad engines in the last decade. Much of engine preference comes down to one of two things:

1. "Ford vs Chevy" type debates.

2. Engine size/HP/torque (with bigger generally being better)

And engine compression brake= jake brake. More effective in braking HP than an exhaust brake or variable vane turbo. Found only on larger engines.

[rhart]

Thank you for your answers Brett. May I ask one more? What is the point at which you need a tag axle on a coach? Seems like a 40 to 42' coach sometimes has a tag but usually does not. Is there a guideline from which to work?

[wolfe10]

Tag axles are used when two axles will not adequately carry the weight of an RV. So, yes, length, weight and "toys" make the difference.

[rhart]

I understand that. Is there a formula I can use to determine if a coach SHOULD have a tag? I'm sure there are some coaches out there that are on the cusp.

[wolfe10]

Depending on model year, there are two terms that will give you an idea of carrying capacity. CCC and NCC are the two terms. The stickers are sometimes in view, and sometimes in cabinets. The terms will be defined on the sticker. Two slightly different ways to giving you an idea of how much of your "stuff" you can add before exceeding the design specs of an RV.

Example, our 38' Alpine (two axle) has a CCC of 4024 pounds. That 4024 pounds is capacity in ADDITION TO full fuel, LP, potable water and 924 pounds for people (done by sleeping positions of 6 times 154 pounds). Fully loaded with our gear, we are still well under GVWR and axle ratings.

[keithannwood]

Cummins 400hp 36000lbs 7.2 running 65mph NO gen while rolling.

We use 7.0 for budget purpose.

[nostlerab]

2009 Knight 41SKQ Cummins ISC 360HP, 8.2 MPG towing 5400lb Jeep GC. We usually travel at 60MPH. Best is 10MPG with a tail wind. Worst is ~6 on the Cassiar in the frost heaves at 30mph.

[bennettjh49]

I completed a 1,119 mile round trip from NW Arkansas to Sioux City, IA. 2003 35' Expedition with the 5.9L 300HP Cummins

9.4 mpg for the entire trip

First 374 miles with a tail wind - 10.7 mpg

Last 745 miles with a head and cross wind and some hills in North Central Iowa - 8.8 mpg

When on the Interstate highways, cruise control was set at 71 MPH.

[DaveAtherton]

Lucky dog 1949, we can go around and around with milage. Unless you can plug Cat ET into your engine ECM and

readout trip data off ECM a person is only guessing. Doing diesel Diagonostic work I read out many,many engine ECM

On both Cat and Cummins engines. What the finding on fuel milage will run from 6.5 mpg to 8.5. mpg. This is engine

only and not taking in genset running. Cummins uses same readout using Insite and milage is same as above.

Dave Atherton Retired Cat Mechanic