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Switching 6X4 3.58rear to 6x2 (with locking dif) 2.64

Discussion in 'Trucks [ Eighteen Wheelers ]' started by Richter, Jun 25, 2013.

Page 3 of 5
  1. Jun 28, 2013 #21
    Freightlinerbob

    Freightlinerbob Road Train Member

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    West Coast B.C.
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    Agreed, but he wants to use a 13 OD and as I pointed out, it likely would not work very well.

    It would help to go to a dealer and ask the salesman to punch in a cruising speed of 60, a 13 with .73 final drive, 2.64 gears and 511 RPM tires. Then tell him you want to haul 80,000# and be able yo manage 6% grades.

    The salesman would fall over backwards laughing.

    But maybe it would all work with one of those magical, non existent, 13 speed direct transmissions that KR mentions from time to time.
     
    Freightlinerbob, Jun 28, 2013
    Freightlinerbob, Jun 28, 2013
    #21
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  3. Jun 28, 2013 #22
    Richter

    Richter Road Train Member

    4,090
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    Feb 13, 2012
    Philadelphia Pa
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    First of all, you have a "low" gear for a reason. By having a low number rear your essentially just running the truck in higher gear all the time. To compensate you use a lower gear on hills. for starts you can even use the "low" gear that now i almost never use. You can get roughly the same tire rpm with a 2.64 in low when compared to a 3.58 in 1st. Unless you regularly use low, it will not affect your low end gearing. As for hills, just drop 1 more gear and your roughly the same tire rpm.

    The advantage is running in 7H which is direct. This will increase MPG. I dont know why your saying it wont because there are numerous articals and case studies saying running direct will. It also out less heat on the trans. At 1400 rpms you can run 63 mph in direct with a 2.64 rear. you would need to be in 8H to dot hat with a 3.58.
     
    Richter, Jun 28, 2013
    Richter, Jun 28, 2013
    #22
  4. Jun 28, 2013 #23
    Richter

    Richter Road Train Member

    4,090
    1,700
    Feb 13, 2012
    Philadelphia Pa
    0
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    Richter, Jun 28, 2013
    Richter, Jun 28, 2013
    #23
  5. Jun 28, 2013 #24
    Richter

    Richter Road Train Member

    4,090
    1,700
    Feb 13, 2012
    Philadelphia Pa
    0
    See above, on the low end its loosing 1 gear not 3. Low with 2.64 is actually slower then 1st with 3.58
     
    Richter, Jun 28, 2013
    Richter, Jun 28, 2013
    #24
  7. Jun 28, 2013 #25
    Freightlinerbob

    Freightlinerbob Road Train Member

    2,234
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    Jan 15, 2012
    West Coast B.C.
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    Running in 7H means you are using the auxiliary portion of the transmission in it's overdrive setting. It is not the same as running a direct transmission in direct. 7H uses the same splitter selection as 8 H or any gear on the high side. In the old days a 13 wasn't labeled with L & H, it was Direct and OD. Nothing has changed, when you're in 7 H you are "overdriving" 7th gear.

    And yes I was wrong about loosing 2 gears and making it like "starting in 3rd" ( I think that's what I said) it is more like loosing 1 gear on the bottom. At any rate, you will have 20% less reduction with a 13 and 2.64 vs. a 10 direct and 2.64.

    I have a 13 and 3.90s and use LO all the time. It's very nice to put so little stress on the drivetrain.
     
    Freightlinerbob, Jun 28, 2013
    Freightlinerbob, Jun 28, 2013
    #25
    04 LowMax Thanks this.
  8. Jun 28, 2013 #26
    BullDoggin

    BullDoggin Light Load Member

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    I run 3.08s with an 18... exactly halfway between 2.64 and 3.55. Top gear is 75mph at 1450rpm, 17th is 64mph at 1450rpm. With a C15 in a brick of a 389, pulling tanks in PA, I'm getting a solid 6.5mpg running 5-7 over the speed limits.

    As you've caught on, Mr. Bob is greatly exaggerating the negatives of your project, especially start ability. Until Jan, I was running my 3.08s with a 13; pulling 80-90k every day... startability might have been an issue with a 318 Detroit and a 120k load, but it just isn't a real world issue for 95% of the trucking world. If you get on LT, talk to Matt Jungbluth. He has done what you want to do, and has a C-16 in a 379EXHD getting 8mpg.
     
    BullDoggin, Jun 28, 2013
    BullDoggin, Jun 28, 2013
    #26
  9. Jun 28, 2013 #27
    D389

    D389 Light Load Member

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    Mar 24, 2013
    Texas Panhandle
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    Would set up like this increase fuel mileage? it is 4 less tires on the ground.
    [​IMG]
    Its hard to see in the pic, but from what I could tell when I walked by this truck was it still had the differential and maybe driveshaft hooked up to the rear axle. I assume it had an airbag set up to lift the axle, I didn't see any chains or straps holding it up. I don't know what the axle ratio was.
     
    D389, Jun 28, 2013
    D389, Jun 28, 2013
    #27
  10. Jun 28, 2013 #28
    Freightlinerbob

    Freightlinerbob Road Train Member

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    It may have had a differential housing but there is no way it had a driveshaft going to it.

    I'm not trying to be overly negative about this, I think I've raised some valid points. If you dig around on KR's forum you'll find a few half ##*ed attempts at this kind of mod that people have not thought through, that ended very badly.

    An 18 speed is a far different trans than a 13 and as I've earlier stated, far better suited to this proposed set up. Because it's lower gears reduce far lower than any OD trans and even as low as a direct. That's what makes it such a great transmission. A gear for every situation.

    As for a C16 getting 8 mpg, I'd like to know what the increase in MPG was. And what kind of job the truck is doing. The OP lists flat deck in his profile, this could likely be 75-80k all the time.


    Furthermore you can't compare the bottom end grunt of a CAT with a Series 60 or Mercedes which is what the OP likely has. Although I don't recall him ever stating what motor he has. Startability should be a concern and should be figured into this modification
     
    Freightlinerbob, Jun 28, 2013
    Freightlinerbob, Jun 28, 2013
    #28
  11. Jun 28, 2013 #29
    BullDoggin

    BullDoggin Light Load Member

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    When's the last time you heard about a truck that couldn't get a load moving. Startability is a non-issue.

    An 18 speed is nearly the exact same transmission as a 13 speed. I'm starting to think you don't have a CLUE as to what you're talking about.
     
    BullDoggin, Jun 28, 2013
    BullDoggin, Jun 28, 2013
    #29
    sdaniel Thanks this.
  12. Jun 28, 2013 #30
    Freightlinerbob

    Freightlinerbob Road Train Member

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    Actually if you look up the ratios for an 18 vs a 13, you will see that there is a vast difference in the overall reduction of the two.


    You are correct that there aren't many, if any, trucks that have startabity issues. But most trucks are specs close enough to do most jobs. What is being proposed here is changing a significant portion of the spec. without addressing the rest of it.
     
    Last edited: Jun 28, 2013
    Freightlinerbob, Jun 28, 2013
    Freightlinerbob, Jun 28, 2013
    #30
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