rear ends.

Discussion in 'Flatbed Trucking Forum' started by thecrow, Apr 24, 2012.

  1. thecrow

    thecrow Bobtail Member

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    Apr 22, 2012
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    hello guys im new to the forum here and i want to learn about gear ratios, i have always driven 355s and a buddy of mine always outpulled me with the same hp and transmission i couldn't figure out why until he burnt them and when he took them apart they were 370s

    so how are they all different?

    390 vs 370 vs 355 vs 336, what are the variables to consider when doing a gear swap or tranny swap or hp increase.
     
  2. Logan76

    Logan76 Crusty In Training

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    I'll throw another wrench into your works, Tall or short rubber.
     
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  3. MNdriver

    MNdriver Road Train Member

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    3.70's have more "leverage" than a 3.55. That's how he can out pull you.
     
  4. Ruthless

    Ruthless Road Train Member

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    The lower the number the less revolutions the driveshaft has to make to turn the wheels. higher number means more power/bottom end with the same hp/tire size/trans etc. lower number means goes faster when you wind her up. higher number means lower top speed and higher rpm to maintain the same speed. somewhere on this site is a link to the eaton road speed calculator.
    I.e. same spec'd truck w 3.08 ratio will go 100mph while a 4.33 will pull houses up hills....very slowly.
     
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  5. thecrow

    thecrow Bobtail Member

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    Apr 22, 2012
    Houston TX
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    so in other words 336 would run like a beauty on flat land and not so good going uphill, where as 390s would be the opposite?

    thanks for the fast replies guys.
     
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  6. MNdriver

    MNdriver Road Train Member

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  7. melpromud

    melpromud Medium Load Member

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    camden ny
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    Well the flat bed forum is the wrong forum for this subject but Ill try to explain anyhow. The number is how many times the drive shaft turns to one turn of your tire. 3.55:1 means the drive shat turns 3.55 times every time the tire turns once. 3.70:1 = 3.7 times. 3.90:1 =3.9 times. 4.10:1 = 4.1 times and so on. The more the drive shaft turns per tire revolution the higher the RPM of the engine at any given speed. The higher the number the lower the gear. 3.90:1 are lower (higher RPMS) than 3.70:1. Every engine has a given RPM that it makes peak torque at. Trucks should be geared so they run in their peak torque at highway speed. Trucks that run heavy or go off road are geared lower than highway trucks. Different size tires will require different gear ratios to have the same RPMs at the same speed. Usually a higher geared truck will get better fuel milage but wont pull as well getting up to speed or pulling up a hill though road conditions, driving style and the type and weight of a load may actually make a lower geared truck get better milage. 3.36:1, 3.42:1, and 3.55:1 are typical highway gears. 4.10:1, 4.30:1, and 4.56:1 are what log trucks, dump trucks, and low boy trucks will typically use. 3.70:1 and 3.90:1 are nice middle of the road gears for trucks that see some heavy haul or off road but also see some long highway miles.
     
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  8. thecrow

    thecrow Bobtail Member

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    Apr 22, 2012
    Houston TX
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    thanks man that was the very answer i was looking for! and as for being on the flatbed section, it was an accident im still learning my way around the site.
     
  9. melpromud

    melpromud Medium Load Member

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    Ya the Trucks(eighteen wheeler) forum is what you wanted. This tread will probably get moved their. The editors of the web sight can do that. Glad my explination helped.
     
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  10. SHC

    SHC Spoiled Rotten Brat O/O

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    [​IMG]


    Sorry, wrong kind. My bad :p
     
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