MPG to expect

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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.
 
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!
 
RV Roamer said:
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!
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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?????
 
Mc2guy said:
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.
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That is great mileage.
 
maryland-david said:
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?????
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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.
 
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
 
RV Roamer said:
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
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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?
 
kevin said:
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 :-\
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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.
 
mdbass said:
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.
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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.
 
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.     
 
"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​
 
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.
 
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.
 
taoshum said:
Why do you care so much about getting the tank full? 
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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​
 
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
 
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.
 
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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