Topic: MPG to expect (Read 5467 times)

RV Roamer · « **Reply #30 on:** July 10, 2009, 02:08:11 PM »

I'm in between you two. We run at 32,700 lbs plus a 5000 lb toad for a total of 37,700 on a Cummins ISL 370. Cruising steadily at 63-64 mph (about 1650 RPMs) on an Interstate, we get 8.1-8.4 mpg. But if I try to do the same on a two lane road, the inevitable variations in speed due to traffic and towns put our average closer to 53 than 63 and the mpg drops to 7.2-7.6, no matter how light I try to keep my foot.

I find that I am not being run over at that speed, far from it. Some people pass me but others seem to be keeping around the same space. I even pass a few.

taoshum · « **Reply #31 on:** July 10, 2009, 07:29:53 PM »

Our last 3000 miles to Nashville and back averaged 8.9 MPG. I set the CC at 65 on the big highways and about 60MPG on the smaller hiways. A headwind makes a large difference compared to a tailwind during the trip. We were towing a Jeep wrangler, our weight was about 27,000 lb counting the Jeep and the engine is a small cummins diesel with a 5 spd Allision AT. I try to keep the tank above the 1/4 level. It was hot, so we ran the generator most of the time to run the house a/c. I did vary the tire pressure (see the other thread) as experiments to improve the ride, plus we have a large elevation change to consider... 400 ft above MSL in Tennessee and 7800 ft elevation at home. I really don't know what the MPG would be for a flat, smooth hiway with no wind, without a/c and a steady 60 MPH... maybe 9-10 mpg?

After a while, the only reason I even look at it is to detect a major leak or problem with the engine or drive train.

Very interesting thread. Thanks. Gary.

BernieD · « **Reply #32 on:** July 10, 2009, 09:30:38 PM »

David

At the last Cummins seminar I attended the presenter said that the "sweet spot" should be around 1650 RPM. I've tried keeping the coach in that area, 1625-1660, and my cruise is set at 62-63 mph. I've averaged tank fills of about 8.75-9.0 mpg and day figures of up to 10. A lot depends on wind, terrain and traffic.

Marsha/CA · « **Reply #33 on:** July 11, 2009, 06:20:55 AM »

Bernie, we are getting just about the same as you with a cat engine. I knew about the "sweet spot" and it seems to work. Tim keeps meticulous records on the coach...(well now that I think about it he keeps meticulous records on everything

) and we seem to run fairly consistent. However on that Gaspe' Peninsula drive we averaged 6.7 mpg something. Those 14-15% grades really made a difference.

Marsha~

maryland-david · « **Reply #34 on:** July 11, 2009, 08:11:03 AM »

Quote from: BernieD on July 10, 2009, 09:30:38 PM

David

At the last Cummins seminar I attended the presenter said that the "sweet spot" should be around 1650 RPM. I've tried keeping the coach in that area, 1625-1660, and my cruise is set at 62-63 mph. I've averaged tank fills of about 8.75-9.0 mpg and day figures of up to 10. A lot depends on wind, terrain and traffic.

What size Cummins do you have? Do you have the Allison tranny? If so is this only 5th gear? I have not put many miles with an automatic transmission so I just let it pick its own gear. In my tractor trailers I always tried to keep the rpm at 1600, but due to the weight of 80,000# or so, it was also necessary for torque. However in 6th gear at 65MPH in my MH, I am way under 1600RPM. Maybe I'll try downshifting and see if that makes a difference.

RV Roamer · « **Reply #35 on:** July 11, 2009, 08:21:41 AM »

Quote

However in 6th gear at 65MPH in my MH, I am way under 1600RPM.

Way under?? You've probably got a larger, torquier engine than I do (an ISM or ISX?), but most Cummins configured for RVs are geared to hit 60 mpg as it shifts into 6th gear at about 1550-1600 rpms.

BernieD · « **Reply #36 on:** July 11, 2009, 09:43:49 AM »

David, I have the 400 ISL. IIRC at the seminar, there were some complaints from owners of the 500-600 hp engines about being able to keep the speed down and the RPMs up. Yes, keeping it in 5th gear was tossed out as an option. You'd have to play with it and see. Do you have an instant or rolling average mpg readout? That could help finding the sweetspot for your engine. That's what I had done and the seminar confirmed my results.

RV Roamer · « **Reply #37 on:** July 11, 2009, 04:11:34 PM »

Guys I know with late model American Heritages (your size of rig) are also seeing 6.x mpg. 2007 and newer engines are also running lower in mpg than those of 2003-2006, at least in the larger engines. Not so much difference at the smaller end (ISB & ISC). Of course, every year the coaches got longer and heavier too!

kevin · « **Reply #38 on:** July 11, 2009, 04:58:25 PM »

Quote

I find that I am not being run over at that speed, far from it. Some people pass me but others seem to be keeping around the same space. I even pass a few.

On our trip to florida and back I never passed anyone while going 65 or 70, I was passed by everyone but some of the poor ole governed truckers... oh well I seem to be getting 6-6.5 running the speed limit.

[edit]fixed broken quote[/edit]

maryland-david · « **Reply #39 on:** July 12, 2009, 06:24:00 PM »

Quote from: RV Roamer on July 11, 2009, 08:21:41 AM

Way under?? You've probably got a larger, torquier engine than I do (an ISM or ISX?), but most Cummins configured for RVs are geared to hit 60 mpg as it shifts into 6th gear at about 1550-1600 rpms.

Gary, I am going to have to check, but I am definitely at least 200-250 under 1650RPM. I understand with the bad fuel mileage of the 2007+, I believe this is due to the DPF. The regens on the DPF diesels do nothing but basically poor raw diesel into the exhaust. My 2008 Duramx diesel gets 2-4MPG less than a nopn DPF diesel. I have just assumed that the Allison tranny was smart enough to choose the right gear. I am going to switch into manual mode and see if my mileage increases from there.

Bernie, I do have the instant MPG gauge, even though I have never thought that they were very accurate, but it is a starting place nonetheless.

On this subject, has anyone noticed changing to synthetic oil in the engine making any noticeable difference in fuel mileage. I only have about 70,000 miles on my coach, so it is not too late to switch to synthetic.

Thanks,

Mc2guy · « **Reply #40 on:** July 12, 2009, 08:43:09 PM »

I might be a bit off topic as I drive a gas engine motorhome, 2009 F53 chassis with the 6.8L V10. I just returned from a three week 5200 mile trip from Virginia to Idaho and back. I would run between 62-66 mph with the generator running 80% of the time and returned 7.3 mpg for the trip.

I saw tanks as high as 8.3 running at night in the high flats of Wyoming, and as low as 5.8 mpg heading into the wind through Kansas.

RV Roamer · « **Reply #41 on:** July 13, 2009, 09:16:48 AM »

David, What engine do you have? I'm guessing a Cummins ISX or maybe a Cat C15? A check of the peak torque range on those engines does indeed show that the sweet spot is closer to 1450 than 1650 rpms, so you probably can cruise at 65 mph at something like 1450 rpms. I stand corrected!

maryland-david · « **Reply #42 on:** July 13, 2009, 09:41:01 AM »

Quote from: RV Roamer on July 13, 2009, 09:16:48 AM

David, What engine do you have? I'm guessing a Cummins ISX or maybe a Cat C15? A check of the peak torque range on those engines does indeed show that the sweet spot is closer to 1450 than 1650 rpms, so you probably can cruise at 65 mph at something like 1450 rpms. I stand corrected!

Gary I have a 2003 Cummins 500. I always just called the an N14e, which I believe stands for 14 liter. Seems like in 2003 it may have been ISM?

?

maryland-david · « **Reply #43 on:** July 13, 2009, 09:47:41 AM »

Quote from: Mc2guy on July 12, 2009, 08:43:09 PM

I might be a bit off topic as I drive a gas engine motorhome, 2009 F53 chassis with the 6.8L V10. I just returned from a three week 5200 mile trip from Virginia to Idaho and back. I would run between 62-66 mph with the generator running 80% of the time and returned 7.3 mpg for the trip.

I saw tanks as high as 8.3 running at night in the high flats of Wyoming, and as low as 5.8 mpg heading into the wind through Kansas.

That is great mileage.

maryland-david · « **Reply #44 on:** July 13, 2009, 10:10:44 AM »

Quote from: maryland-david on July 13, 2009, 09:41:01 AM

Gary I have a 2003 Cummins 500. I always just called the an N14e, which I believe stands for 14 liter. Seems like in 2003 it may have been ISM??

I just assumed since my last 500HP Cummins from the 1990's was an N14e that this one was also, maybe it is an ISX, I'll check and get back to you.

BernieD · « **Reply #45 on:** July 13, 2009, 05:57:51 PM »

David

It is probably the ISM. The ISX was up around 600hp.

RV Roamer · « **Reply #46 on:** July 13, 2009, 06:07:31 PM »

According to Cummins the current ISX's are 14.9L and may be configured anywhere from 385 hp to 600 hp (usually 525 or 600 in an RV). But I suspect you are right that in 2003 they were using an ISM rather than the ISX. ISMs are 10.8L and can be anywhere from 280 hp to 500, but in RVs they are usually 450 or 500 hp, since the ISL delivers 370-425 hp. Peak torque can be either 1450 or 1550 on an ISM, depending on the configuration. David seems to be running at 1450. That's why I suspected an ISX, but an ISM is a possible too.

http://www.cummins-sp.com/document_library/Spec_Sheets/Engines/ISM_500.pdf

maryland-david · « **Reply #47 on:** July 13, 2009, 06:22:57 PM »

Quote from: RV Roamer on July 13, 2009, 06:07:31 PM

According to Cummins the current ISX's are 14.9L and may be configured anywhere from 385 hp to 600 hp (usually 525 or 600 in an RV). But I suspect you are right that in 2003 they were using an ISM rather than the ISX. ISMs are 10.8L and can be anywhere from 280 hp to 500, but in RVs they are usually 450 or 500 hp, since the ISL delivers 370-425 hp. Peak torque can be either 1450 or 1550 on an ISM, depending on the configuration. David seems to be running at 1450. That's why I suspected an ISX, but an ISM is a possible too.

http://www.cummins-sp.com/document_library/Spec_Sheets/Engines/ISM_500.pdf

Well Gary this is what I get for assuming. I had a Cummins 500 in a tractor trailer and it was a N14e, I assumed that's what mine was. I looked at the data plate on the engine today, boy was that a pain to get to! It is indeed an ISM rated at 500HP at 1800RPM. Peak torque is at 1200RPM. I took it out on the road and at 65MPH I am between 1050-1100. I will try 5th gear and 1200RPM and see if that increases my fuel mileage at all for a trip and rerport back.

Any known issue or known problems with the ISM's?

RV Roamer · « **Reply #48 on:** July 14, 2009, 09:44:38 AM »

No issues that I know of.

mdbass · « **Reply #49 on:** July 16, 2009, 07:54:58 AM »

Quote from: kevin on July 10, 2009, 12:22:24 PM

Not to start a fight, but how on earth do you not get run over on the interstate? you must be doing30 on the big hills too? I guess if that is what ya gotta do to get 9-11mpg then so be it,but I just can't run that slow $:-\$

No offense taken-us "non banks" 235 hp DP guys see 30 on hills occasionally. Like I said, we stay to the left. Let me tell you I'd love a 400-600 hp motor-but this is what I could afford. I may have the governor adjusted or purchase a Banks kit eventually.

maryland-david · « **Reply #50 on:** July 16, 2009, 07:11:01 PM »

Quote from: mdbass on July 16, 2009, 07:54:58 AM

No offense taken-us "non banks" 235 hp DP guys see 30 on hills occasionally. Like I said, we stay to the left. Let me tell you I'd love a 400-600 hp motor-but this is what I could afford. I may have the governor adjusted or purchase a Banks kit eventually.

If you are in the US, I hope that you stay to the right, not the left! I would be very haqppy with your fuel mileage and don't worry about your speed! Just try n ot to be a hazzard.

On my previous post of RPM's, that was a typo, that was my RPM at 55MPH in 6th gear. Most of the time it wouldn't let me slow down to 55 in 6th gear, but I got it to do it a couple of times. At 65MPH I am about 1375RPM. I may try a trip at 60MPH in 5th gear and see if that improves my mileage at all.

HJohsens · « **Reply #51 on:** July 28, 2009, 12:01:31 AM »

I have a 1995 Safari with an 8.3 liter Cummins that I have been getting 7.5 MPG so far. My driving has ranged from 55 MPH in the Peoples Republik of Kalifornia (where I live) to 75 or 80 MPH in Texas. I am not consistent at all. I have just changed the intake air system so that it is less restrictive and I hope to get better MPG next week when we drive to LA.

My experience is that I was lied to regarding what MPG I could expect when I bought it (which I fully expected) and that most RV'rs will pad their mileage when talking to others, including other RV'rs.

I was told (or perhaps "sold" to expect 10 MPG or more) but my 7.5 mpg is a grand total of over 6000 miles driven divided by the amount of fuel pumped into the tank. I have burned out the tank to near empty at times and spent a huge amount of time filling the tank to make sure I am full-up and topped off.

With diesels it is important to know that the diesel fuel foams up significantly when being pumped. For this reason before filling my tank I will use the levelling jacks to raise the filling corner of the tank to the highest point (so that the foam will vent out through the filler neck). I will also spend the time waiting for the foam to settle out and fill the tank to capacity.

In my experience I have found that depending on climatic conditions where I am filling I have had the pump shut off when I have just over 3/4 of a tank full.

DonTom · « **Reply #52 on:** July 28, 2009, 12:33:12 AM »

"75 or 80 MPH in Texas."

Expect a lot of forum members here to chew you out for going at such speeds in a RV!

I once mentioned going 75 MPH in my small 22 foot RV on a rather empty Colorado interstate freeway ( where the speed limit was 75 MPH) and boy, did I get jumped on here in this forum!

-Don- SF, CA

taoshum · « **Reply #53 on:** July 28, 2009, 06:11:45 PM »

Why do you care so much about getting the tank full? For me, I have to stop every 300 miles anyway for a break or I just get tired and stop for a while, sometimes several days... anyway I have plenty of chances to fill the tank... even if it goes to 3/4 or whatever, I don't care... besides it's less weight to carry. The only time I get concerned is if we are stopping someplace without electricity and I want to make sure the gen/set has plenty of fuel.

It's obviously up to you but it seems extreme to lower the jacks to get a foam free fill?

my 2 francs. thanks, G.

taoshum · « **Reply #54 on:** July 28, 2009, 06:15:34 PM »

I forgot to mention... my long term average is about 8.5 but we "only" have 300Hp and the only time I see 75 is going downhill when I can see another uphill coming right away and I don't want to get runover by the big truck that is right behind me. LOL, whew. Thx, G.

RV Roamer · « **Reply #55 on:** July 28, 2009, 06:30:37 PM »

My sentiments as well. We seldom feel the need to cover maximum miles or see how far we can get on a tankful. Relax and enjoy is our motto.

DonTom · « **Reply #56 on:** July 28, 2009, 07:28:07 PM »

Quote from: taoshum on July 28, 2009, 06:11:45 PM

Why do you care so much about getting the tank full?

It helps keep the fuel pump in the gas tank cooler so it will last longer. It's also better for the tank to keep it full as much as possible to prevent rust from forming.

-Don- SSF, CA

Mark R. · « **Reply #57 on:** July 28, 2009, 08:27:51 PM »

Weight does not effect MPG all that much unless you have to travel uphill both ways, this link is worth looking at and printing it is great for arguments like this one. Enjoy,

http://www.holtrv.com/pdf/understanding.pdf

GKman · « **Reply #58 on:** September 07, 2009, 10:36:27 AM »

We just finished a 2600 mile trip from Missouri to Utah and Santa Fe with a 5.9 (360) gasoline 1999 Dodge conversion van pulling a 23' Rockwood fiberglass over aluminum stud trailer with a GVW of 4660 (if we had it loaded to factory specs). Average mileage was 8.3 mpg with the worst on I-70 at 70 mph of 6.9. Fuel cost was $0.32/mile. Dodge has a 3.55 rear end and was driven with the OD locked out. A lot of shifting down to 2nd. I think gearing around 3.9 would eliminate a lot of the down shifting. Nice, comfortable, dependable van with room in the back for the 4-wheeler for about 1/2 of it's Cash-for-Clunkers value, got the job done.

HODAK1172 · « **Reply #59 on:** September 14, 2009, 09:37:34 PM »

I just purchased a 08 f350 powerstroke w/ 4:10 gears last week. I love the truck and the power is great. I got rid of my gas 2500 Dodge and was expecting a huge gain in MPG. However, thats not the case. I tow a 09 Sydney Outback 5er, around 10k lbs. I just went about 2 hrs from my house and averaged 9.5 mpg towing. City driving, unloaded, I am at 13.5. This is my first Diesel and want to know how these numbers stand compared to others. My gasser was getting 8 towing and 12 city, unloaded. At least I have a little bit of a gain!!!

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Author Topic: MPG to expect (Read 5467 times)