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Direct drive bus gear ratio

Discussion in 'Trucks [ Eighteen Wheelers ]' started by Swadian, Oct 28, 2018.

  1. Oct 28, 2018 #1
    Swadian

    Swadian Light Load Member

    184
    51
    Jan 21, 2017
    Reno, NV, USA
    0
    I have a MCI 102D3 long-haul bus, curb weight 30500 lb, GVWR 44400 lb, used as express bus in Nevada on long desert highway drives (regular long-distance route, NOT a charter bus). Engine is Detroit Diesel 6067WK28 (Series 60 11.1L 365hp). Transmission is Allison HT746 4-speed automatic, direct drive in 4th gear. Current gear ratio is 3.21:1. Tire size is 315/80R22.5.

    I'm going 70 mph all the time here and I feel like the engine RPM is way too high at these speeds: frequently getting to 1700 RPM or above. I want to bring down the RPM. Should I replace gear ratio with 2.64:1 or 2.79:1 to save fuel? How much might it cost, and where can I get it done?

    Again, I'm 80-90% of the time running at 70 mph on the highway. Engine is in great shape but I'm frustrated with the RPM. Any advice?

    Please move to appropriate forum section if this is inappropriate.
     
    Swadian, Oct 28, 2018
    Swadian, Oct 28, 2018
    #1
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  3. Oct 28, 2018 #2
    x1Heavy

    x1Heavy Road Train Member

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    White County, Arkansas
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    Do you have access to the temperatures of your engine, trans, and axles? If all are happy at 1700 then you have a little room to run the upgrades common in the west.

    It's not much difference than a full 2000 500+ Detroit at 1650 and 63 in my time. To me that's slow in RPM, I started with trucks that wind out to 2300 with a variety of engines and ungoverned in my time. All of the engines with a couple of exceptions have done well. Later engines redlined at 1950 or 2100 depending on what you had. And there isnt much useful horses or torque on the engine chart at those levels. Just speed.

    Detriots like to run, but are not like Cats. They can however lug when it's time to do it. That means coming down from 1600 towards 1200 maybe before taking the next gear down. They work hard to give up the RPM's in that situation.

    My answer is food for thought. It is not a solution or adding to the problem. Gear ratio is not my forte, but if you are running 1700 at 70 or whatever out west, that's nice. As long all of your temperatures are happy.
     
    x1Heavy, Oct 28, 2018
    x1Heavy, Oct 28, 2018
    #2
  4. Oct 28, 2018 #3
    Ezrider_48501

    Ezrider_48501 Road Train Member

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    Apr 2, 2011
    bismarck, nd
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    its been a long time sense iv run an 11.1 detroit but i seem to remember them liking to be in the 14-1800 rpm range, there are some calculators online you can find in google search plug in your rear end ratio and speed and tire size i would probably want to target to be around 1500 rpms at 70. that bus was probably originally setup to cruse at 60-65
     
    Ezrider_48501, Oct 28, 2018
    Ezrider_48501, Oct 28, 2018
    #3
    wore out and magoo68 Thank this.
  5. Oct 28, 2018 #4
    Banker

    Banker Road Train Member

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    Aug 19, 2012
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    I run low pro 22.5’s on a car carrier with a Series 60 12.7 Detroit, 2:64 and 18 speed. I run in 16 up to 65 at roughly 1550 rpm. I run in 17 up to 75. I am not sure with your transmission how this would compare to my setup. Eaton has a great online tool, road speed calculator. Maybe it will help with your setup but I am not sure with your transmission.
     
    Banker, Oct 28, 2018
    Banker, Oct 28, 2018
    #4
  7. Oct 29, 2018 #5
    magoo68

    magoo68 Road Train Member

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    st malo mb canada
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    That 11 litre may not be happy with 2.64 or 2.79 .. rpm may be too low for a low hp low torque engine from that era .. talk to a knowledgeable Detroit rep and see what they recommend .. there’s very few people here who have ever run a 11 liter series 60
     
    magoo68, Oct 29, 2018
    magoo68, Oct 29, 2018
    #5
    wore out Thanks this.
  8. Oct 29, 2018 #6
    Swadian

    Swadian Light Load Member

    184
    51
    Jan 21, 2017
    Reno, NV, USA
    0
    A small correction, it now seems like the bus is running at nearly 1800 at 70 (redline at 2100). It has a 4-speed auto Allison HT746 so I can't really shift it much. Mechanic at W.W. Williams says I should bring it down to 1500 for better MPG, but I don't know how knowledgeable he is about a Series 60 11.1L.

    The engine and trans temp are fine, not sure about axles. The manual says "ENGINE TEMP" warning is not applicable to DDEC and says I should use 315/80R22.5, so I don't think I can put in larger tires because the wheel wells are closed in. I can't see much unless I go over a pit.

    Used an online calculator and found that the bus is geared for 60 mph at 1500. According to the calculator, 2.64 would get me to 70 at 1500. So if the mechanic is right, I should replace with 2.64. All of this has just become very confusing.
     
    Swadian, Oct 29, 2018
    Swadian, Oct 29, 2018
    #6
  9. Nov 1, 2018 #7
    WitchingHour

    WitchingHour Road Train Member

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    Broomfield, CO
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    315s on a drive axle? Seems like a really wide footprint for a bus.
     
    WitchingHour, Nov 1, 2018
    WitchingHour, Nov 1, 2018
    #7
    x1Heavy Thanks this.
  10. Nov 4, 2018 #8
    Swadian

    Swadian Light Load Member

    184
    51
    Jan 21, 2017
    Reno, NV, USA
    0
    That's what it says on the build plate and manual:
    • Steer Axle: 315/80R22.5
    • Drive Axle: 315/80R22.5
    • Tag Axle: 315/80R22.5
    Everyone uses it on this model bus.
     
    Swadian, Nov 4, 2018
    Swadian, Nov 4, 2018
    #8
  11. Nov 5, 2018 #9
    Swadian

    Swadian Light Load Member

    184
    51
    Jan 21, 2017
    Reno, NV, USA
    0
    Anything else on the gear ratio?
     
    Swadian, Nov 5, 2018
    Swadian, Nov 5, 2018
    #9
  12. Aug 24, 2020 #10
    someguy99

    someguy99 Bobtail Member

    1
    0
    Aug 24, 2020
    0
    Hi Swadian.
    Did you ever change the gear ratio on your bus ?
     
    someguy99, Aug 24, 2020
    someguy99, Aug 24, 2020
    #10
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