3.70 vs 3.91 gear ratio comparisons? Whats the difference?

Discussion in 'Ask An Owner Operator' started by Thotphobia, Aug 3, 2021.

  1. Thotphobia

    Thotphobia Bobtail Member

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    Apr 21, 2021
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    Sup drivers, I’ve been on the lookout for a semi. We really want a 3.70 gear ratio, but there’s so many trucks with 3.91. What’s the difference between them if they both have an x15 with 565 hp 1850 torque? Is the 3.70 gear ratio faster? Please help. Thanks!
     
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  3. Magoo1968

    Magoo1968 Medium Load Member

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    St Malo mb
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    My truck is 3.70 with 22.5 tires and a coworker has 3.90 with 24.5 tires we both turn around 1600 rpm at 70 mph . If you drive fast I’d go 3.70 or 3.55 but if you run heavy like I do the 3.70 works nicely.
     
  4. Dino soar

    Dino soar Road Train Member

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    I don't know about that engine but pretty much if you're out west and you're going to do 75 and 80 to 370 is probably better.

    If you pull super heavy loads and you're really going through some mountains the 390s will pull better.

    It's a different situation because I have a big cam Cummins, but at 75 miles an hour with a .73 overdrive and 22.5 tires and 390 rears at 75 I'm turning I think about 1700 RPMs.

    The 370 rears will drop the RPMs I guess a few hundred RPMs and 24.5 tires will also drop the RPMs.

    Depends what you're looking to do with the truck.

    The road ranger Road speed calculator will show you what your RPMs will be depending on your transmission and tire size and rear axle ratio.
     
  5. Cat sdp

    Cat sdp . .

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    Orion's Belt
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    Difference is almost nill depending on rubber……

    I got 3.70 gears with 11R24.5 rubber and haul over 100K gross every load……
     
  6. Thotphobia

    Thotphobia Bobtail Member

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    Apr 21, 2021
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    We will have 24.5s and we don’t do hills. We’re in NC. Nice flat straight roads local hauling max miles pero load 60
     
  7. Dino soar

    Dino soar Road Train Member

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    I would think the 390s would be great for that. You have to see what the manufacturer tells you the cruising RPM should be and then check with roadranger Road speed calculator.

    With 22.5 tires my truck runs like 1400 or so at 60 miles an hour. You'll run a little less than that with the 24.5 Tires.
     
  8. Ruthless

    Ruthless Road Train Member

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    The City.
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    3mph if all other spec is the same in final od at 1500 rpm
     
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