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Old 05-20-2012, 07:33 AM   #1
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RPMs at 60 mph ??

New to me 2004 Dutch Star model 3809 on a Spartan chassis with 370 hp Cummins / Allison 6 speed.

I keep reading about people running 62-63 mph at 1650 rpms. They keep talking about 1650 rpms being the sweet spot. I am not seeing anything close to that as I see 1800 at 62 mph according to my gauges. I did not have the trans in "Economy Mode", but I don't think that changes the rpms.

I read on another forum where poster claimed they had to get to 68mph to shift into 6th gear and then could back off to 65 mph. If that is the case, I never got to 68 as I'm of the 60-62 mph comfort zone type of driver. Perhaps I was in 5th gear? I will call Spartan on Monday to see if they can tell me what rear end ratio I have. I think it is a 4:88.

Am I the only one looking at those kind of numbers? What is your targeted rpm for maximum economy?
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Old 05-20-2012, 07:54 AM   #2
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Sounds like a little confusion because most mhs shift into sixth gear at 58 mph. Sometimes you have to go to 60 mph and let it shift then backoff if you want 55 mph. THe Economy Mode will help it stay in sixth gear at 55 mph because it will change the downshift point on hills so it doesn't downshift as much.

"Sweet Spots" vary from engine to engine and even the year model of the same engine. My Detroit "Sweet Spot" is 1420 rpm. Some engines run at higher RPMs than others. Normally the smaller the engine the higher the rpm required because they are not torque monsters like the big engines. Most mh are designed to run best at 62-63 mph. You should be able to listen and watch the tach and count the shifts and see if you are in sixth gear at 58-60 mph. The other thing to try is on a level road at 62 mph hit the downshift button to bring it down into 5th and see if the tach goes up and you hear it downshift. If you are still in 5th and do that then nothing will happen.

Sweets Spots are all good and well but wind resistance/drag is what kills and mh's MPG. Being in sixth gear at 62-63 mph is a good compremise in MPG and not irritatng the other vehicles on the road by going real slow.

Let us know what you find out.
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Old 05-20-2012, 10:39 AM   #3
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Sounds like a little confusion because most mhs shift into sixth gear at 58 mph. Sometimes you have to go to 60 mph and let it shift then backoff if you want 55 mph. THe Economy Mode will help it stay in sixth gear at 55 mph because it will change the downshift point on hills so it doesn't downshift as much.

"Sweet Spots" vary from engine to engine and even the year model of the same engine. My Detroit "Sweet Spot" is 1420 rpm. Some engines run at higher RPMs than others. Normally the smaller the engine the higher the rpm required because they are not torque monsters like the big engines. Most mh are designed to run best at 62-63 mph. You should be able to listen and watch the tach and count the shifts and see if you are in sixth gear at 58-60 mph. The other thing to try is on a level road at 62 mph hit the downshift button to bring it down into 5th and see if the tach goes up and you hear it downshift. If you are still in 5th and do that then nothing will happen.

Sweets Spots are all good and well but wind resistance/drag is what kills and mh's MPG. Being in sixth gear at 62-63 mph is a good compremise in MPG and not irritatng the other vehicles on the road by going real slow.

Let us know what you find out.
I have never understood the term "Sweet Spot". I know what you are talking about but I would think the "Sweet Spot' would be the RPM that gives peak engine torque. That info can be obtained from the engine data. I have a 1997 Beaver Marquis with a 425 HP Cat. On the torque curve, the peak is at 1150 RPM. The RPM, as displayed by my Silver Leaf, is a little over 1150 RPM to achieve the 6th gear shift point, about 55 mph. On my last trip, Yuma AZ to central Washington state, I set the cruise control at that point for the entire trip. Milage was 9.4 mpg. My next trip, starting in a couple of days, i'm going to bump the speed up to 60 mph. This should eliminate any guess work. I think 60 mph is a good speed and is legal in all states except California.

Jim E
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Old 05-20-2012, 10:40 AM   #4
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I also have a 370 hp Cummins ISL in an 04 Freightliner-based chassis and I run about 64 mph at 1600 RPMs. My rear axle is a 4.63.

As Mike described, the Sweet spot can vary quite a bit, so don't be surprised if yours is different than someone elses.
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Old 05-20-2012, 10:45 AM   #5
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The sweet spot is typically somewhere between peak torque rpm and peak horsepower rpm, with the peak torque end of the range being most common. Just where it falls will vary a lot depending on the engine torque/hp curve, coach weight and air resistance, transmission and rear axle gearing, tire rolling resistance, head/tail winds, and probably a host of other small variables.
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Old 05-20-2012, 11:55 AM   #6
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It has always been my understanding that "Sweet Spot" only pertains to the motor and is as already said about halfway between the peak torque RPM and the Peak HP RPM. Once the Sweet Spot for a motor is known then the overall gear ratio (tire diameter, rear gear) is designed to run the vehicle at the speed you want at that RPM. It just so happens my Deytoit Sweet Spot of 1450 RPM is right at 62 mph and I will bet you that is the way most mhs are designed to run somewhere between 62 and 65 mph at the motors best and most economical RPM.
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Old 05-20-2012, 12:48 PM   #7
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Planocat, what size tires do you have on your mh? My '05 DSDP with the 370 came equipped with 275/70 series tires. After switching to 275/80 series my rpm drop from 1800 to 1700 at 62 mph so 60 mph is in the 1600's. My fuel mileage improved when I made switch.
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Old 05-20-2012, 01:17 PM   #8
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Quote:
Originally Posted by Planocat View Post
New to me 2004 Dutch Star model 3809 on a Spartan chassis with 370 hp Cummins / Allison 6 speed.

I keep reading about people running 62-63 mph at 1650 rpms. They keep talking about 1650 rpms being the sweet spot. I am not seeing anything close to that as I see 1800 at 62 mph according to my gauges. I did not have the trans in "Economy Mode", but I don't think that changes the rpms.

I read on another forum where poster claimed they had to get to 68mph to shift into 6th gear and then could back off to 65 mph. If that is the case, I never got to 68 as I'm of the 60-62 mph comfort zone type of driver. Perhaps I was in 5th gear? I will call Spartan on Monday to see if they can tell me what rear end ratio I have. I think it is a 4:88.

Am I the only one looking at those kind of numbers? What is your targeted rpm for maximum economy?

Here is a link to your MH if needed. It shows a 4:88 rear end ratio.
http://www.newmarcorp.com/uploads/br...20Brochure.pdf
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Old 05-20-2012, 01:43 PM   #9
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Bragadoon:

I think you have nailed my problem with tire size. I had been looking at a 2001 40' Dutch Star also on a Spartan chassis. It had 275-80 R 22.5 on it and when I decided on this 38' that was 200 miles away, I foolishly assumed it also had 275 80's. Needing new tires, I had ordered 6 to be replaced when I got it home. Much to my surprise it has 275-70's on it. I called Newmar and they said yes, that was the correct size and didn't recommend changing it.

Did you change out rims, speedo calibration, etc when you upgraded the size? That was my first thought but both Newmar and the tire dealer don't seem real keen on the idea and I think clearance would be an issue.

Things went downhill in a hurry as the tire dealer says if I do decide to increase to the 80's, it will require different (wider) rims and wouldn't put the 80's on my current aluminum rims. He also took away my allowance for the old tires as he claims he has no use for the 70's as it is an oddball size...... there goes $300 he would have given me if I had 80's.

Then I drive it home and see the increased RPMs, which are probably due to the tire size. I was anything but impressed with the fuel economy and would guess it is tied to increased RPMs over what everyone else is seeing.

Close look at the tires and it doesn't appear I have the clearance for the additional 2" of heighth that comes with the 80's if I did decide to change rim/tire size. Guess I'm stuck with the 275-70-r22.5 and the additional RPMs that I don't want.

I was willing to pay the additional price for Michelins until the old tire allowance was pulled and I'm now back in the search for tires I can afford. I guess the Continentals will get the call as we're talking $1000 difference between the 2 brands.

Thanks for all the responses to what seems to be an unsolvable problem. Live and learn.
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Old 05-20-2012, 01:44 PM   #10
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I think the significance of the "sweet spot" concept is greatly exaggerated IMHO. As your speed increases air resistance becomes the dominant factor in your fuel economy. My CAT C-12 is set to shift into 6th at ~62-63; I find that cruising at just above that speed is comfortable for me and the drivetrain. If I use the Economy setting on the Allison I can avoid most of the 5th gear downshifts that would normally accompany cruising this close to the shift point. If you're interested in understanding how speed, weight and engine size affect fuel economy you might enjoy reading http://weisstravels.com/resources/CA...er+reduced.pdf
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Old 05-20-2012, 03:36 PM   #11
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Regardless there is a Sweet Spot for each motor. On diesel submarines with diesel electric motors they are run right on the Sweet Spot so they get the best MPG. It makes a big difference for them because they run at a constant RPM.I do believe that wind resistance plays a bigger part in mpg than anything else other than the MH weight.
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Old 05-20-2012, 07:23 PM   #12
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Everyone is throwing "Sweet Spot" around. Would someone please explain "Sweet Spot" and how it is determined? I assume it is the engine RPM that results in the highest MPG. Why isn't that at the engine peak torque point?

Jim E
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Old 05-20-2012, 08:03 PM   #13
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Everyone is throwing "Sweet Spot" around. Would someone please explain "Sweet Spot" and how it is determined? I assume it is the engine RPM that results in the highest MPG. Why isn't that at the engine peak torque point?

Jim E
I read the book that the engine designers took the time to put together for me. (owners manual) For my engine, Cat C9, they state repeatedly, at least 6 times in 3 pages of how to run the engine, that the most efficient (efficient is the word they use each time) way to run the engine is to run it at peak torque. They don't suggest peak HP, or somewhere in between peak torque and peak HP.

They don't use the words sweet spot anywhere in their literature. The owners manual for my Ford Lehman in my boat likewise never uses that technical term. They use the term efficient and the phrase, "for best efficiency". Both engines seem to be very happy running at the rpm the engineers say is most efficient. I call that the sweet spot. I've never driven even a fraction of "most of the MH's" on the road today, so I rely on the folks that design and build them.

PS, my MH shifts to 6th at 62 mph.

Ken

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Old 05-20-2012, 08:14 PM   #14
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So Ken at what rpm is your peak torque at and do you think you could efficiently drive it at that rpm? My peak torque is at 1200 rpm and my advertised Sweet Spot is at 1420 rpm which is 62 MPH. There is no way I can drive it at 1200 rpm. My torque curve is pretty flat but my transmission will not let me drive at 1200 rpm at a steady speed
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