Ideal cruising RPM for MP8?

Discussion in 'Experienced Truckers' Advice' started by Bdog, Feb 4, 2021.

  1. Bdog

    Bdog Road Train Member

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    I have a Mack day cab with the MP8 505HP / 1860 TQ and an 18 speed with 11R24.5 rubber. Gears are 3.79. I am in Texas and most roads I drive on are 75 MPH limit and some 80. Doing 78 MPH in 18th it is turning 1,700 RPM. I’m thinking it would be much better down around 1,300 RPM? The truck drives great and 90% of the time I gross around 65k with a flatbed. I’m thinking maybe I should regear? My fuel mileage stinks. I have never got out of the 4’s. Would getting the RPMs down help a lot? I have noticed I am in the upper half of my gears at 15 MPH. Seems I have a ton of gears down low but need a higher one. This truck isn’t on the road every day or even every week but most trips are 3-600 miles.
     
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  3. AModelCat

    AModelCat Road Train Member

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    Considering how little you plan on/are using it, I think a re-gear would take a very long time before you see a return on investment.
     
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  4. Bdog

    Bdog Road Train Member

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    You may have a valid point. Another issue is i can’t drive a full ten hours without needing fuel. It would be really nice if I could extend my range a bit. Not sure how much a rehear costs but if I could get 6MPG that would save me $150 in fuel per 600 miles - maybe $600 per month - $7,200 per year
     
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  5. 86scotty

    86scotty Road Train Member

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    1700 RPM? Way too high IMO. I'd slow down too but that's just me. I rarely run over about 73.
     
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  6. Bdog

    Bdog Road Train Member

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    Slowing down isn’t really an option. I travel in a convoy with a bunch of pickups to job sites and they want to go the speed limit.
     
  7. Moosetek13

    Moosetek13 Road Train Member

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    A 3.08 rear might give you the RPM you want at cruise, but what would that do to getting up to speed?
    You might need to rethink every gear in the tranny as well.
    And don't forget the camshaft in the motor, to give better torque at the lower end.


    I once had a car that got much better mileage at 90 than it did at 65.
    It seemed the engine liked the higher RPM's, so I geared it up from a 3.08 to a 4.10.
    Better off the line, but the mileage wasn't that much better.
     
  8. Bdog

    Bdog Road Train Member

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    I’m no expert I’m really asking here but my observation is I have 18 gears and use half of them by the time I am going 15 mph and am turning too many RPM at highway speed. Seems like I could gear higher and have plenty of low gears? As it is now I only ever split between 17th and 18th.
     
  9. AModelCat

    AModelCat Road Train Member

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    1700 really isn't all that high to be honest.
     
  10. Moosetek13

    Moosetek13 Road Train Member

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    Then you may only need to change that rear gear to something lower in number, like a 3.08.
    That would give you a bit more range in each gear on the low end and lower the RPM's at the higher end.
     
  11. Joyce's Volvo

    Joyce's Volvo Light Load Member

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    A 18 sp is same as13sp only difference is you can split low side ( low range) . In stead of changing rears you my want to change trans , could go to a double od trans. It could be less expensive but, you'll lose the low hole that you have now.
     
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