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Projects Gear in the rear

Discussion in 'The Hokey Ass Message Board' started by steamer, Jan 13, 2013.

  1. steamer
    Joined: Aug 17, 2008
    Posts: 199

    steamer
    Member

    What does the gear ratio translate to 4:11, 5:14 etc......? Is one for fast start or top end or what. It's confusing.......
     
  2. Don's Hot Rods
    Joined: Oct 7, 2005
    Posts: 8,319

    Don's Hot Rods
    Member
    from florida

    What it means is the ratio is actually 4.11 to 1. The driveshaft will turn 4.11 times for every time the axle shaft turns the wheel. The higher the number (numerically) the deeper the gears are, which means they are more suited to launching the car as opposed to top end.

    A 2.75 to 1 rear would be considered "high" and more for freeway driving to lower the rpms of the motor. A 4.11 to 1 is considered "lower" and is more suited to performance off the line usage.

    Confusing, isn't it ??? :D

    Don
     
  3. gimpyshotrods
    Joined: May 20, 2009
    Posts: 16,929

    gimpyshotrods
    ALLIANCE MEMBER

    It is 4.11:1 and 5.14:1, expressed as ratios. So, for a 4.11:1 ratio, it takes 4.11 turns of the driveshaft to make one turn at the axles.

    The higher the number to the left of the colon is, the "lower" the gear ratio is.

    The lower the gear ratio is, the quicker the launch, but the lower the top speed.

    The higher (numerically lower) the gear ratio, the slower the launch, but the higher the top speed.

    Given enough power, a car with a 2.50:1 rear gear might go 200mph at redline, but take a while to get there. The same car, with a 5:14 might turn a 10.00 second 1/4 mile, but top out at 125.
     
  4. Don's Hot Rods
    Joined: Oct 7, 2005
    Posts: 8,319

    Don's Hot Rods
    Member
    from florida

    To make it a little easier to understand, let's use round numbers. If you had a car with 4.0 to 1 gears and the motor was turning 2,000 rpms, the axle would be turning at 500 rpms. If you had 2.0 to 1 gears in the rear at the same 2000 rpms the axles would be going around at 1,000 rpms.

    It would seem that the higher gear ratio (numerically lower) 2.0 would be faster, and they might at top end, but you have to get the mass of the car moving from a dead stop and the engine would be lugging to get that done with the 2.0 ratio. The 4.0 will get the car moving much quicker.

    Don
     
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  5. gimpyshotrods
    Joined: May 20, 2009
    Posts: 16,929

    gimpyshotrods
    ALLIANCE MEMBER

    Remember as well, tire height plays a role in this equation too. While it does not change gear ratio, the circumference of the rear tires determines how far you go in one turn of the tires. Putting taller tires on the same vehicle can make it "seem" like it has higher gears, and shorter ones, lower.
     
  6. Fenders
    Joined: Sep 8, 2007
    Posts: 3,922

    Fenders
    Member

    And if you are looking at gas mileage and highway driving, the 2.0 will be better.

    If you want a quick launch in a short race, the 4.0 will be better but worse gas mileage on the highway and higher RPMs at highway speed.
     
  7. steamer
    Joined: Aug 17, 2008
    Posts: 199

    steamer
    Member

    Thanks to you all, that makes it more understandable.
     
  8. Don's Hot Rods
    Joined: Oct 7, 2005
    Posts: 8,319

    Don's Hot Rods
    Member
    from florida

    A while back there was a discussion about this same subject and it seems there is a difference in the interpretation and terminology from region to region.

    Where I grew up, "low" gears were like 4.56, and "high" gears were like 2.75's. But some people said they were taught exactly the opposite. I guess it all depends on what your friends were calling them.


    Don
     
  9. F&J
    Joined: Apr 5, 2007
    Posts: 13,217

    F&J
    Member

    same here
     
  10. gimpyshotrods
    Joined: May 20, 2009
    Posts: 16,929

    gimpyshotrods
    ALLIANCE MEMBER

    Of course, the weight of the vehicle is a factor in choosing a rear end gear, too. Given the same engine, transmission, and tire size, a heavier car will need a somewhat lower gear to gain advantage to get the mass of the vehicle moving, whereas a lighter on could use a higher one, and get the same result.

    The top gear of the transmission is also a factor in the choice. A transmission with a 1:1 top gear might go well with a 3.00:1 rear gear, and one with a 0.72:1 overdrive might do just as well with a 4.11:1. Given the same equipment, otherwise, the cruise RPM would be about the same (give or take a few RPM), but the launch would be quicker on the overdrive car.
     
  11. gimpyshotrods
    Joined: May 20, 2009
    Posts: 16,929

    gimpyshotrods
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    Ain't physics and math fun?
     
  12. Moon Rocket
    Joined: Dec 26, 2012
    Posts: 540

    Moon Rocket
    BANNED
    from GA

    "Where I grew up, "low" gears were like 4.56, and "high" gears were like 2.75's. But some people said they were taught exactly the opposite. I guess it all depends on what your friends were calling them."



    On that basis I guess they call 1st. gear in a three speed trans high and third gear low? :rolleyes:
     
  13. Don's Hot Rods
    Joined: Oct 7, 2005
    Posts: 8,319

    Don's Hot Rods
    Member
    from florida

    Nope. The gear ratio of first in any transmission is "lower" than the ratio of the top gears. So we call that one "low gear". :D Same with the rear end...........at least in my neck of the woods. :p

    Don
     
  14. Rusty O'Toole
    Joined: Sep 17, 2006
    Posts: 9,068

    Rusty O'Toole
    Member

    In a typical transmission "Low" might be 3:1 while "High" would be 1:1. Just to make things more confusing.
     
  15. Moonequipt13
    Joined: Jul 9, 2012
    Posts: 196

    Moonequipt13
    Member

    Tall or high = numerically lower (2.75, 3.0)
    Low or short =numerically higher (4.11, 5.29)
     
  16. Fenders
    Joined: Sep 8, 2007
    Posts: 3,922

    Fenders
    Member

    I think he was being facetious.
     

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