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RobRV · 11-26-2009, 03:23 AM

I'm wondering where the Sweet Spot is ? I would like to achieve the Best fuel economy while still maintaining decent forward momentum.
Our coach a Newmar DS 38 Feet is New to Us and drives and runs very well. We will be taking off soon on an Extensive Trip South from BC to Mexico . Our route will take Us down the I5 . The Speed limit is generally 55 MPH , I would like to keep up with whatever the Flow of traffic allows, but wonder if I'm doing the right thing staying at or near 55? Our Speedometer has an error (Reads 55 ~actual 57 GPS) the Tachometer reads only 1700 RPM at this speed.
We seem to be keeping pace with the Truckers but are being passed all the time by other RV's that seem to be traveling at 10~15 MPH faster.
Should I be driving a bit quicker or am I doing the right thing by trying to Go with the Flow!
I have noticed that 1900 RPM seems to bring Me into the 65 MPH Range, is this too Fast ?
What about maintaining momentum on Steep grades? I would hate to Lug it , but is there a danger of Over Revving by downshifting ?
I guess I just need to hear what You folks Do!

DBOL · 11-26-2009, 05:20 AM

Cummins 8.3 Allison 3000?is this right

FleetMan · 11-26-2009, 08:06 AM

No matter what the rig,I try to cruise at approximately 300 rpm above peak torque rpm. That way, no lugging the engine and my speed is normally about 62 mph. Each rig, engine and overall gearing will vary.

Steep grades, don't lug, down shift manually to a gear which will allow you to accelerate going up hill if you need to speed up. Hold that rpm and watch all your temps.

J Walker · 11-26-2009, 08:29 AM

I believe the answer to your question will come from running at two or three max speed and checking your MPG. I don't have an 8.3, but my 5.9 ISB likes 55 MPH max a whole lot better than 65 MPH, by about 2-3 MPG. On paper 65 MPH is better from a peak torque standpoint. In practice maybe that is not true. From a wind resistance and rolling resistance standpoint, 55 MPH is a whole lot better than 65 MPH.

slabman · 11-26-2009, 08:42 AM

For a comparison, my 500HP ISM has taller gearing and I run around 60 mph/1350RPM. It seems to take the grades quite well without bogging down and fuel mileage peaks in the high 50's. The one thing you'll find is that the faster you go, the more the wind resistance on your windshield will "push back", lowering your fuel mileage.

So, keep it slower and don't let others around you dictate your speed.

GaryKD · 11-26-2009, 08:47 AM

Hi Rob,
Go to Cummins PowerSpec. Choose "vehicle gearing". Follow the menus to get the answer to your question.

For me, (ISC 330), I cruise at 60 MPH. This is 1750 RPM. Cummins wants me to maintain 1800 RPM minimum. I tried this and the fuel mileage went down. 1750 seems to be the best RPM for fuel mileage while moving a comfortable pace.

Peak torque, for my engine is 1350 RPM.

mythplaced · 11-26-2009, 09:00 AM

I'm one of the "1750 club"

400 ISL 3000 Allison

68 mph 8.5 mpg

That's my story, and I'm sticking to it!

RobRV · 11-26-2009, 09:55 AM

Quote:

Originally Posted by DBOL

Cummins 8.3 Allison 3000?is this right

Yes!

It is a 1998 Newmar..Dutch Star Diesel Pusher.
The Engine is a C 8.3 Non Electronic Cummins
The Transmission is an Allison 3060 Series 3000 Six Speed

The Consensus here seems to be 1750 RPM which would put Me at the 58~60 MPH Range, I will try to maintain that on the trip this Winter. I will keep the Log Book up to date !

It seems to Me that this will be reasonable and should make for a relaxing drive .

Thank You for the quick response! Anyone with any other Hints is welcome to post!

Ronnie Bledsoe 2006 Dutchstar 43' · 11-26-2009, 10:10 AM

I have the ISL 400 Allison 3000. Live in Ca where the speed limit is 55 when towing. I drive at 58-60MPH a tad over 1700Rpm. A little over 40K lbs towing a 4600 lb Jeep and I get 7.1 MPG.

Jeff and Cheryl · 11-26-2009, 10:29 AM

RobRV,
I have the same engine/transmision set-up you do. I have found that 1500 RPM in 6th provides the best fuel economy. This equates to about 55 MPH. However, I usually drive at about 1625 RPM which is about 60 MPH. Towing a Honda CRV I am getting 9 MPG + or - depending upon road and traffic conditions. Rain and head winds kill fuel economy. 60 MPH is a good compromise between keeping up with traffic and maximizing fuel economy. 55 is the posted speed limit in CA for vehicles towing, however the truckers mostly do 60 to 65 and the cars are doing 75 to 80. Hope this is helpful.

J Walker · 11-26-2009, 07:26 PM

Gary, Thanks for the Cummins Power Spec link. Going through it tells me I am below the lower RPM band a bit at 55 MPH. I will do some more experimenting. FWIW I changed two things at once when I lowered my speed to 55 MPH from 65MPH, and the second one was that I began to use Power Service Diesel Fuel treatment. I found a 2+ MPG increase in MPG and I am not sure whether it is the speed decrease or possibly cleaning the fuel injectors. I will continue to use Power Service and perhaps try increasing the speed some.

deandec · 12-07-2009, 04:42 PM

When driving our C8.3L Cummins, I travel at about 59 MPH on cruise control. Weight is 34,000 lbs with toad. At that speed in CA, I am flowing with the big trucks so seldom have to gear up to pass traffic I am following. Makes for a pleasant drive with less stress.

I think the rig is at about 1650 RPM at that speed but I do not drive by RPM readings.

clubmed98 · 12-07-2009, 05:25 PM

I do about 55-60 on cruise control..seems to be happy and i pull a crv and weigh about 32,000..i also have the upgraded 275 tires..so i might be closer to 55 mph...higher speed wind resistance probably a bigger cause of lost mpg than a slightly wrong ideal rpm..just mho

Roadking · 12-11-2009, 10:47 AM

Rob: Do you know if your tranny has shifted into 6th gear yet at 55mph? I have the ISC 350 and Allison and in flat level terrain with just moderate acceleration my tranny will not shift into 6th until 57mph. Once in 6th I can then slow to 55 but if there is any incline then the Allison downshifts to 5th. If I keep my speed at 58mph or higher then the Allison will stay in 6th and does not upshift/downshift as much. If I'm terrain where it shifts a lot I will manually downshift to 5th until the terrain flattens, or I speed up and the Allison will then hold in 6th.

In my rig there is about 350 RPM difference between 5th and 6th. In 5th at 57mph I run approximately 1750 RPM and the same speed in 6th is approximately 1400 RPM. I try to keep in the 1400-1500 RPM range in 6th which is about 57 to 60 mph.