Best gear ratio for 15 speed

Discussion in 'Volvo Forum' started by 416autohauler, Mar 1, 2024.

  1. 416autohauler

    416autohauler Bobtail Member

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    Mar 1, 2024
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    Hi guys, I have 2006 Volvo with a d12 and an eaton 15 speed direct transmission, I don’t think it has overdrive. It has a gear ratio above 4 but not completely sure. I haul a 5 car hauler so my loads are usually light and I drive on the highway in Ontario Canada where roads are majority of the time flat. What gear ratio do you guys recommend I change to as my trucks rpm’s are way to high at 100km or 60 miles. Which gear ratio should I change it to comfortably run at 100km and still pull a hill. Thanks
     
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  3. Magoo1968

    Magoo1968 Road Train Member

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    Those years of d12’s weren’t very strong that’s why someone geared it that way.. I doubt if you pick up much for fuel mileage . It will take many years to break even , your best time to regear is when one rearend needs parts replaced..
     
    AModelCat Thanks this.
  4. uncleal13

    uncleal13 Road Train Member

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    If the rear ratio is in the 4’s then it is definitely an overdrive. A direct would be geared something under the 3’s.
    I had a 15 over with 3.73 rears and a series 60. It worked pretty well.
    The 15 over had a top gear ratio of 0.79 vs 0.73 for a 13 or 18 speed.

    Joel Morrow said even the old D12 was a European engine designed to run really low rpms. With your lighter loads it would probably work well with something in the low 3’s for a rear ratio. Need to know tire sizes to get better numbers. Or maybe ask him on facebook.
     
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