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Old 06-17-2017, 12:44 AM   #1
Mini-Skoolie
 
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Join Date: Apr 2017
Location: San Francisco
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Year: 1974
Coachwork: Crown
Chassis: Supercoach - Single Axle
Engine: Cummins NH-250, Fuller T-905M
Overdrive on a Crown Supercoach: How High is High Enough?

In reading the manual on my Fuller T-905 transmission I was sort of amazed to see that it can be rebuilt as an overdrive unit, which would change 5th gear from 1.00 to somewhere around 0.65.

I say somewhere around 0.65 because I have the M gear set, and while the M is listed, it is not listed in the table on overdrive ratios.

And amazed because that's what I love about nice, old, rebuildable, versatile machines.

Which brings me to the real question, what's the right transmission for my bus? Figure I'm optimizing for 1) fuel efficiency and 2) fun and easy to drive anywhere.

The truth is, the jump from 1st two second is about 1,000 RPM, which is too big in my opinion. Going up any kind of grade it's hard to get to second gear before the vehicle slows down too far.

The rest are 500 RPM apart, and she goes 62 mph at 2,000 rpm in fifth. If I rebuilt to an overdrive at 0.65 then I'd be down at 1250 RPM at 62 mph. But would she cruise at 62mph@1250RPM. Maybe because I'm nearly empty and not hauling a few thousand pounds of human flesh.

I'd been thinking a ten speed was the answer, but the real question is which ten speeds?

Another night I'll sit down and do the math. Here are the ratios for the deeply curious.

[IMG][/IMG]

Welcome your thoughts,

Adam
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Old 06-17-2017, 12:45 AM   #2
Mini-Skoolie
 
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Year: 1974
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Engine: Cummins NH-250, Fuller T-905M
And here's speed and torque curve.

[IMG][/IMG]
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Old 06-17-2017, 01:20 AM   #3
Mini-Skoolie
 
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Year: 1974
Coachwork: Crown
Chassis: Supercoach - Single Axle
Engine: Cummins NH-250, Fuller T-905M
Here's what the numbers say:

[IMG][/IMG]

Which is to say I should get 4.7 with current gearing.

With overdrive and cruise down at 1,300 RPM (if that's even possible), I would still be at 6.9 mpg. By percentage that's a big improvement, but I was hoping for more like 10 - 12 mpg.

There's always a chance this set of curves isn't actually what I'm running.
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Old 06-17-2017, 02:17 AM   #4
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You need an RTO-910.
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Old 06-17-2017, 08:53 AM   #5
Mini-Skoolie
 
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Year: 1974
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Chassis: Supercoach - Single Axle
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I see now that the images don't show on phone. They do show in browser in full site though. Will debug later.


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Old 06-17-2017, 08:57 AM   #6
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The numbers you ran were all at full load. 62 MPH will be the same load, determined by frictional and aerodynamic load, say 150 HP, not full load at the various RPM points you're looking at.
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Old 06-17-2017, 08:59 AM   #7
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Join Date: Apr 2017
Location: San Francisco
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Year: 1974
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Chassis: Supercoach - Single Axle
Engine: Cummins NH-250, Fuller T-905M
Rameses - RT0-910 is what I was thinking. Looks like Overdrive ratio on that is 0.81. Fuller has the length on the 910 at 28.9" which is a little longer than the 27.6" for the T-905.


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Old 06-17-2017, 09:05 AM   #8
Mini-Skoolie
 
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Join Date: Apr 2017
Location: San Francisco
Posts: 52
Year: 1974
Coachwork: Crown
Chassis: Supercoach - Single Axle
Engine: Cummins NH-250, Fuller T-905M
Ennonne - yes, I pulled those off the web. Full load is hopefully higher fuel consumption than whatever load I have at 62.

Time to get my own numbers.


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Old 06-17-2017, 03:19 PM   #9
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Join Date: Jan 2016
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Year: 1976
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I went through this very sme thought process with my Crown. Rebuild the T-905M (that's what I currently have) or get the rto-10. The manual only states that the t-905a and 905b can be rebuilt to have overdrive, I'm not sure about all of the other 905 models. But, just like you, I decided a 10 speed way to go.
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Old 06-17-2017, 08:25 PM   #10
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Join Date: Jun 2016
Location: Orange County, CA
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Year: 1990
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Engine: Detroit 6V92TAC, DDEC 2, Jake brake, Allison HT740
Rated Cap: 37,400 lbs GVWR
Quote:
Originally Posted by upbeat View Post
Here's what the numbers say:

[IMG][/IMG]

Which is to say I should get 4.7 with current gearing.

With overdrive and cruise down at 1,300 RPM (if that's even possible), I would still be at 6.9 mpg. By percentage that's a big improvement, but I was hoping for more like 10 - 12 mpg.

There's always a chance this set of curves isn't actually what I'm running.
I think you should be getting a lot more than 4.7 MPG. A friend of mine's Crown tandem with a Big-Cam Cummins and RTO is getting about twice that! I don't know what differential gearing he has, bus I suspect it's higher than Crown's usual 4.1 - he can easily cruise at 75 MPH, with plenty more to spare. His was a Needles CA bus, so it probably has much higher overall gearing than most for the long runs across the desert. (It also has factory A/C!) Here's a short video of Brian test-driving it, pretty well for not having driven a 10-speed for a while:

John
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