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Topic: Coach speed vs. RPM (Read 862 times) previous topic - next topic

Coach speed vs. RPM

In order to keep my engine RPM above 1500 at a reasonably safe speed, I can never use 6th gear. In 6th gear above 1500 RPM I would be at 80 MPH or faster and I am not comfortable driving that fast. Anyone else have this problem?
Royce & Denise, MC #17410
'01 U320 4220 ISM450 Build #5895 SOLD
Toad: '10 Honda CR-V

Travel makes one modest. You see what a tiny place you occupy in the world. - Gustave Flaubert

Re: Coach speed vs. RPM

Reply #1 –
Hmmm. 1500 in mine is right at 65

2000 u-320 4010
Tim Fiedler    2025 LTV Unity MBL on Order
2000 Chevy Tracker 2 Door Convertible 4WD Now lifted 4.5"
1985 Citroen 2CV6 Charleston
Murphy Rebel on wheels with 175HP Titan
Cessna P337
1980 48' Westport MY (!/4 Share)

Re: Coach speed vs. RPM

Reply #2 –
I am just over 1500 RPM at 70 MPH in 6th.
David and Carolyn Osborn
1995 U320C SE 40' Build 4726 Feb 1995
FMCA 147762
Motorcade 17186

Re: Coach speed vs. RPM

Reply #3 –
I don't like to let the RPM drop below about 1700 because it feels like it's lugging in 5th at that RPM.
Royce & Denise, MC #17410
'01 U320 4220 ISM450 Build #5895 SOLD
Toad: '10 Honda CR-V

Travel makes one modest. You see what a tiny place you occupy in the world. - Gustave Flaubert

Re: Coach speed vs. RPM

Reply #4 –
Royce,

Wonder if your tach is accurate? 

Could have it verified.

Could use this formula to verify: (Engine RPM x 60) / (Rear Axle Ratio x Tire Revolutions per Mile x Transmission Ratio) = MPH
Brett Wolfe
EX: 1993 U240
Moderator, ForeForum 2001-
Moderator Diesel RV Club 2002-
Moderator, FMCA Forum 2009-2020
Chairman FMCA Technical Advisory Committee 2011-2020

Re: Coach speed vs. RPM

Reply #5 –
96 U295. C8.3 Cummins. Allison 3000 series. 1800 rpm=62 mph 6th gear
Steve & Ginny Hill
96 U295 36'/Wrangler
Tampa

Re: Coach speed vs. RPM

Reply #6 –
Quote from: BamaFan –
In order to keep my engine RPM above 1500 at a reasonably safe speed, I can never use 6th gear. In 6th gear above 1500 RPM I would be at 80 MPH or faster and I am not comfortable driving that fast. Anyone else have this problem?

Tach or speedo seems off
"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: Coach speed vs. RPM

Reply #7 –
1700 rpm seems a little high, max torque is at 1200 rpm, believe cummins recommends 1500 rpm for best all round performance.
At 1500, I am going 70.5 mph by silverleaf and gps.
Jim C.
coachfree, previous 1997, 1999, 2000, and 2003 Foretravels.

Re: Coach speed vs. RPM

Reply #8 –
Royce, we same set up.  We right at 1375 rpm at 65 mph.  No need to be 1500 rpm, IMHO. My reading on rpm not by digital though so tach a little hard to read very closely but the 1375 seems it.
Mike
2001 U320 4010 Build 5878 (Gus)
Wrangle Unlimited Toad
Nacogdoches

Re: Coach speed vs. RPM

Reply #9 –
I remember seeing a chart somewhere indicating optimal fuel economy is at 1400 on the ISM so that is where I keep it.  Either 55mph in 5th or 63-64mph in 6th.

Re: Coach speed vs. RPM

Reply #10 –
Quote from: jcus –
1700 rpm seems a little high, max torque is at 1200 rpm, believe cummins recommends 1500 rpm for best all round performance.
At 1500, I am going 70.5 mph by silverleaf and gps.
That is closer to what I found. To keep 1500 rpm I have to be over 71 mph.
The selected media item is not currently available.Barry BEAM #16014
2003 U320 40' AGDS
Beamalarm, Foretravel technical help and specifications
"Whatever the mind can conceive and believe, it can achieve"

Re: Coach speed vs. RPM

Reply #11 –
Different coaches will have different rear end ratios so different speeds for a given RPM. To verify tachometer, use Silverleaf or Pro-Link 9000 and it will give you a digital readout to compare to the analog tach reading.

The Allison should sense a "lugging" condition and shift down to keep RPM within specs for the load.

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

Re: Coach speed vs. RPM

Reply #12 –
Cummins will have a spec that says "above XX RPM full throttle operation allowed". Now, this was mostly for mechanical engines. The computer will prevent overloading (aka "lugging") of the M11.

For flat land driving, I'd aim for around 1300-1400rpm at cruise of 70-75mph. Just kick it down to 5th for the hills.

I'm re-gearing my c8.3. Going to go from 5.13 to 4.33. There's a thread I made about it.
1998 U270 34'

Re: Coach speed vs. RPM

Reply #13 –
Pierce said:
Quote
The Allison should sense a "lugging" condition and shift down to keep RPM within specs for the load.

So from what I am reading here is 1500 is not too low an RPM for a ISM450. Correct?
Royce & Denise, MC #17410
'01 U320 4220 ISM450 Build #5895 SOLD
Toad: '10 Honda CR-V

Travel makes one modest. You see what a tiny place you occupy in the world. - Gustave Flaubert

Re: Coach speed vs. RPM

Reply #14 –
I think it is peak torque for that engine, the effiecency sweet spot
Tim Fiedler    2025 LTV Unity MBL on Order
2000 Chevy Tracker 2 Door Convertible 4WD Now lifted 4.5"
1985 Citroen 2CV6 Charleston
Murphy Rebel on wheels with 175HP Titan
Cessna P337
1980 48' Westport MY (!/4 Share)

Re: Coach speed vs. RPM

Reply #15 –
1500 rpm, best combination of power and efficiency.
Jim C.
coachfree, previous 1997, 1999, 2000, and 2003 Foretravels.

Re: Coach speed vs. RPM

Reply #16 –
I went to find the chart that I found before regarding ISM fuel consumption vs rpm and all I can find the is the one for the C83 which does indicate 1500 is the sweet spot.  I could swear what I found before was that 1400 was the sweet spot for the ISM.  Either way I don't think it has nearly the effect that cruise speed does (55mph vs 65mph).

Re: Coach speed vs. RPM

Reply #17 –
Sweet spot is always max torque rpm per cummins RV mileage publication, as far as engine efficiency goes. Mind you that may not be a speed/top gear sweet spot depending on gearing
Tim Fiedler    2025 LTV Unity MBL on Order
2000 Chevy Tracker 2 Door Convertible 4WD Now lifted 4.5"
1985 Citroen 2CV6 Charleston
Murphy Rebel on wheels with 175HP Titan
Cessna P337
1980 48' Westport MY (!/4 Share)

Re: Coach speed vs. RPM

Reply #18 –
Quote from: piku –
  Either way I don't think it has nearly the effect that cruise speed does (55mph vs 65mph).

Agree. Going from 55mph to 65mph requires more HP. Making more HP requires burning more fuel!

What a engine fuel efficiency curve shows is how efficient the engine is at making each of those HP. Brake Specific Fuel Consumption (BSFC).

So, your engine may consume more fuel if it is putting out more HP, but actually operating more efficiently.
1998 U270 34'