Skip to main content ↑ ↓
Topic: Optimum RPM range for longivity and economy (Read 727 times) previous topic - next topic

Optimum RPM range for longivity and economy

I'm a new owner and was reading through the manuals when I came across information that stated 1800 rpm's to be ideal and it is best to select a gear to keep you close to this rpm. Following this guideline, 5th gear is about 1700rpm's and 70mph. Not sure when you would use 6th. Does anybody else do this? Or did I miss read?
Brad & Melanie
2003 U320 3820 1st timer
'97 TJ and/or '85 CJ7

Re: Optimum RPM range for longivity and economy

Reply #1 –
That may be true if you were climbing significant grades.  You wouldn't be going 70 mph in any case.  Higher RPMs under load are going to increase cooling.  65 mph in 6th on your transmission will put the ISM11 at about 1400 rpm.  That is pretty close to the performance/fuel economy sweet spot.  If you are on reasonably flat terrain run in 6th gear at whatever speed you are comfortable with.  I like to be under the speed limit enough to avoid the pass/re-pass game with trucks.  Much less stress, better fuel economy.
Roger Engdahl and Susan Green
2001 U320 3610 #5879 (Home2) - 2014 Jeep Cherokee or 2018 F150
Hastings, MN

Re: Optimum RPM range for longivity and economy

Reply #2 –
I don't worry about it: the only thing I DO keep a eye on is the max rpm/ temps for our coach. With the great combos FT has with the Allison six-speed, no concerns: the Allison will figure it out!  ^.^d  The biggest problem I had when we got this coach, after two 'slow-pokes', was looking down and realizing, "I was doing 85." !
1993 U-240 "La Villa Grande"..CAT 3116 w/ Pacbrake PRXB...Allison 3060 6-speed..
Previous: 1983 Airstream 310 turbo diesel, 1979 Airstream 280 turbo diesel
                                      Build # 4297
                                      PNW natives
                      Home base:  'Cactus Hug' (Ajo, Arizona)
                        DW Judy & Chet the wonder dog
                        Full-Timers 'Sailing the asphalt sea'

Re: Optimum RPM range for longivity and economy

Reply #3 –
3126 Cat, 1700 rpm's is the sweet spot. Best mileage 8+, and happy speed for me. 62 MPH

" I am looking at the wold through a windshield, watching life fly by me on the right"......Commander Cody
Chris and Tammy White  CDA Idaho
Previous owners 1997 U295 36' 3126 Cat 300 HP Build # 4998
Former Foretravel tech & RVIA certified tech
Former owner Custom Satellite home/RV satellites 
Former owner Vans LTD  van conversions
Unemployed, panhandler, drag racer NHRA #6348

Re: Optimum RPM range for longivity and economy

Reply #4 –
I agree with Mike - for most normal flatland cruising conditions, just pick the speed where you are comfortable, and let the Allison brain box worry about selecting the gear.  In almost all cases, it will make the correct choice.
1993 U280 SE 40' WTBI, Build: 4359
C8.3 300hp, 6-Speed, Exhaust Brake
960 watts on the roof (6 x 160)
Sorento (or BOLT) on a Kar Kaddy SS
"Nature abhors a vacuum"

Re: Optimum RPM range for longivity and economy

Reply #5 –
Quote from: gracerace –
3126 Cat, 1700 rpm's is the sweet spot. Best mileage 8+, and happy speed for me. 62 MPH
I had to check ours a couple of times, so I was not ranked with a 'fish story': 3116 CAT.....12 mpg....75 mph/45 on 7% grades, it  doth not care: what a sweet combo!  b^.^d
1993 U-240 "La Villa Grande"..CAT 3116 w/ Pacbrake PRXB...Allison 3060 6-speed..
Previous: 1983 Airstream 310 turbo diesel, 1979 Airstream 280 turbo diesel
                                      Build # 4297
                                      PNW natives
                      Home base:  'Cactus Hug' (Ajo, Arizona)
                        DW Judy & Chet the wonder dog
                        Full-Timers 'Sailing the asphalt sea'

Re: Optimum RPM range for longivity and economy

Reply #6 –
Ifixboats...I completely agree with Roger....according to your signature your coach should have the M11 ISM450...the max torque RPM for this motor is 1450RPM.  4 stroke Diesels perform best at peak torque.  68MPH is where I hit 1450 RPM, and, for greatly increased fuel economy and longevity, I stay below that in level cruise as much as possible.  Agreed, you'd never want to be in 5th gear intentionally in cruise, unless you just like buying diesel or your tranny has an issue.  Select D and go, Allison 4000 will not up-shift to 6th until 60mph+.  Use ECON mode for other than 60 slightly earlier up-shift points.  Up long mountain passes, I will down-shift so as to keep the RPM at or a little above 1450RPM with accelerator barely off the floor...just keep an eye on coolant and tranny temps on long up-hill grades.  Just my $.02.  Woody.

Quote from: ifixboats –
I'm a new owner and was reading through the manuals when I came across information that stated 1800 rpm's to be ideal and it is best to select a gear to keep you close to this rpm. Following this guideline, 5th gear is about 1700rpm's and 70mph. Not sure when you would use 6th. Does anybody else do this? Or did I miss read?
2001 4010 U320 build #5865 "Bluto-d-Bus" since 09/18
2006 Honda Element ESP Toad
Full timing since 2016 in Western MT
Copilot: Sitka
1975 GMC 260 Avion: sold

Re: Optimum RPM range for longivity and economy

Reply #7 –
Quote from: mike leary –
I had to check ours a couple of times, so I was not ranked with a 'fish story': 3116 CAT.....12 mpg....75 mph/45 on 7% grades, it  doth not care: what a sweet combo!  b^.^d
You got that pointy nose helpin'....LOL
Chris and Tammy White  CDA Idaho
Previous owners 1997 U295 36' 3126 Cat 300 HP Build # 4998
Former Foretravel tech & RVIA certified tech
Former owner Custom Satellite home/RV satellites 
Former owner Vans LTD  van conversions
Unemployed, panhandler, drag racer NHRA #6348

Re: Optimum RPM range for longivity and economy

Reply #8 –
Quote from: mike leary –
I don't worry about it: the only thing I DO keep a eye on is the max rpm/ temps for our coach. With the great combos FT has with the Allison six-speed, no concerns: the Allison will figure it out!  ^.^d  The biggest problem I had when we got this coach, after two 'slow-pokes', was looking down and realizing, "I was doing 85." !

Amen on the 85!  Found myself in that spot for a moment 2 weeks ago outside of Tucson.  It can get away from you if you're not paying attention.....They sure ride nice!    (Yes...I slowed down!)

Dave
Dave & Diane
2001 U270 36' WTFE #5820
2016 Jeep Wrangler Hard Rock
Motorcade #18156

Re: Optimum RPM range for longivity and economy

Reply #9 –
It is good to have/know the torque curve for your engine.  I like to go into a climb on the high side of peak torque so that as you slow down going up hill, you will drop down into the peak torque RPM which will help keep up your speed.  If you start up hill at peak torque, you;ll start loosing speed and torque and steadily decline.  Engines all vary in this regard, so know your torque curve and use it to your advantage.  Have a great day  ----  Fritz
Fritz & Kathy Johnson
1991 36'

Re: Optimum RPM range for longivity and economy

Reply #10 –
My m11 not ism has gotten 8.2 mpg driving it hard when needed at 69 mph for the last 12k miles at 36,000 pounds combined.

Torque "feel" and horsepower revving it out are not a lot different. 

But that's with a resonator not the oem muffler and easy rolling Michelin tires
"Riding and rejoicing"
Bob
1997 U320 40' Mid entry, build 5132,  wtbi ce27, 4th owner
2007 Solara convertible
2 prodeco tech outlaw ss electric bikes

1095 watts solar
08 Ls 460 and a sc430
2000 Ford F-250 superduty 4x4
 

Re: Optimum RPM range for longivity and economy

Reply #11 –
Maximum engine life is not about treating your diesel with kid gloves. It's about managing engine temps while climbing grades and downshifting to allow the water pump to do it's job plus keeping the EGTs down. It's also about not idling your engine for extended periods, and cooling the oil temperature down before shutting down, especially at the top of grades where the oil will coke and ruin the turbo bearings and seals. Read the engine manufacturer's recommendation and you can't go wrong.

I beat both the U300 and MBZ 300SD hard but make sure the above is followed plus change the oil. The U300 has over 100,000 trouble free miles and the 300SD is approaching 450,000 miles without even a water pump replacement. Same for the transmission. When pulling a trailer, the 300SD is petal to metal for much of long distance trips. Still runs and starts like new even in sub-freezing temps.

Pierce
Pierce and Gaylie Stewart
'93 U300/36 WTBI
Detroit 6V-92TA Jake
1140 watts on the roof
SBFD (ret)