Problem with choosing the right rear axle ratio

Discussion in 'Questions To Truckers From The General Public' started by Hoffa, Jun 5, 2021.

  1. Hoffa

    Hoffa Bobtail Member

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    Hello all,
    I own a 2008 Freightliner m2 106 medium duty rollback tow truck with a Cummins 6.7 ISB and Allison 1000/2000 4th Gen. I have a problem with my RPM’s going up soo high at 55mph (2400 rpm’s) I took the truck to a Freightliner dealer for inspection and all they could tell me was that the rear axle 5.86 ratio was my problem. Is there anyone out here who can advise me on what to do or what can be the problem or what size rear axle ratio I need to upgrade/downgrade to? Please help I drive over 300 miles a day at 55mph on 65-70 mph highways which it’s very annoying and also concerning since I’m worried i’de be forcing the engine if I pass over 2500 rpm’s for long periods.
    thanks in advance for any advice!
     
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  3. AModelCat

    AModelCat Road Train Member

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    There is no problem, the truck is simply not setup correctly for your application.

    Go online and punch in your truck specs into an RPM calculator (Spicer has a pretty good one). Just play with different rear end ratios until you get the cruising RPM where you want it.
     
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  4. Roberts450

    Roberts450 Road Train Member

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    Are there any codes? Is the transmission actually getting into high gear?? There should be a sticker on the drivers door jam to verify the rear axle ratio and the tire size that the truck was manufactured with. 24-2500 isnt all that hard on the 6.7 as its rated speed is up around 3000rpm.
     
  5. Hoffa

    Hoffa Bobtail Member

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    No codes at all. From the invoice Freightliner provided they did not find any problems/codes so they suggested that the problem could be the rear axle ratio.
     
  6. Hoffa

    Hoffa Bobtail Member

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    89FC9393-1523-4A16-8407-117A760FA941.jpeg
     
    Last edited: Jun 5, 2021
  7. Hoffa

    Hoffa Bobtail Member

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    From the info I gather at spicer website the rpms should less going at 75mph with my truck setup 5.86 rear axle ratio 22.5 tires 5 speed transmission
     
  8. Roberts450

    Roberts450 Road Train Member

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    What size tires to you have?
     
  9. Roberts450

    Roberts450 Road Train Member

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    Also need to know exactly which transmission you have to determine what gear ratio’s it has.
     
  10. Hoffa

    Hoffa Bobtail Member

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    Jun 5, 2021
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    Tires 255/70 R22.5
    Transmission Allison 1000/2000 4th Gen
     
  11. Flint1

    Flint1 Medium Load Member

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    Sounds like the op has 19.5" rubber. Being as it is set up as a roll back tow truck, someone may have installed 19.5" rubber to lower the unit.
    I own several units all with Allison 2500 transmissions and 3 different axle ratios.
    All on 11R22.5 rubber
    5.29 at 55mph is 1750rpm
    5.57 55mph is 1850 rpm
    6.17 55mph is 2100 rpm.
    All in 5th gear. Allison 1000/2000 have nearly the same ratio in the top 2 gears..

    Most allison 1000/2000 of that era only had 5spds available, Allison disabled 6th unless the manufacturer engineered for 6th, allison won't just allow 6th gear without it.

    Use an online calculator too see what you have. 5th gear in the allison should be 0.71

    Edit: op posted the 255/70r22.5 after I posted, which are lo pro 22.5s..
    6.7 isb in medium duty is 2500rpm max
     
    Last edited: Jun 5, 2021
    AModelCat Thanks this.
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