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Technical Gear Ratio Question

Discussion in 'The Hokey Ass Message Board' started by gregsmy, Mar 3, 2020.

  1. gregsmy
    Joined: Feb 11, 2011
    Posts: 46

    gregsmy
    Member
    from Florida

    So I am making time to work on my project 36 Ford truck. Couple hours in the evenings after family stuffs over and whenever I have extra time on the weekend. When I bought the truck the po had set the drivetrain from a 1987 Lincoln MarkVII LSC in the frame and thats about how far it got. After putting the body together I figured out that the rear axle is really about 3" to wide to get the tire and wheels to fit in the back fenders. He had the LSC wheels on the back that are offset and if you put a more typical wheel on it they are past the edge of the fender. So after looking around I have decided on a Explorer rear axle. The LSC had the 200hp 5.0 roller motor in it that year with a 3.27 rear gear ratio. My plans are not to do much with the engine at this time so no big horsepower increases. I was reading some where that a better gear ratio is a good way to make a low horsepower vehicle a little peppier. The Explorer seems to have to standard ratios, 3.55 or 3.73 with the 3.73 being limited slip in most cases. Right now I have things mocked up with a 29" tall tire on the back. I am leaning towards finding a limited slip with the 3.73 ratio but the 3.55 seem to be easier to find. So with that is there any big advantage to one ratio over the other?
     
  2. fastcar1953
    Joined: Oct 23, 2009
    Posts: 2,134

    fastcar1953
    Member

    153 rpm. powerwise won't be able to notice.
     
  3. pitman
    Joined: May 14, 2006
    Posts: 4,920

    pitman

    3.55 will serve fine.
    If your desire, is to
    'race for pinks' or
    other questionable
    practices :eek: the taller
    the ratio, the quicker
    Out of the Gate!
     
  4. dan31
    Joined: Jul 3, 2011
    Posts: 1,045

    dan31
    Member

    Check out Wallace racing calculators. Rpm /mph
     
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  5. trollst
    Joined: Jan 27, 2012
    Posts: 1,726

    trollst
    ALLIANCE MEMBER

    You'll be happy with 355.
     
    lothiandon1940 likes this.
  6. Should factor in what transmission, and it's first and final ratios, to optimize your rear ratio pick.
     
    mad mikey and pitman like this.
  7. Boneyard51
    Joined: Dec 10, 2017
    Posts: 4,019

    Boneyard51
    Member

    A lot depends on how and where you are going to drive the vehicle the most. If you are just going to drive on the street, at 60mph, go 3:73 if you are going to spend a lot of time on the interstate at 75+mph go with the 3:55.






    Bones
     
    pitman likes this.
  8. irishsteve
    Joined: Jan 10, 2017
    Posts: 730

    irishsteve

    What transmission are you using,and is it overdrive? Whats the OD ratio? 3:55 with a tall tire,and lots of OD ,and you could end up with a final ratio around 2:11 .
     
  9. gregsmy
    Joined: Feb 11, 2011
    Posts: 46

    gregsmy
    Member
    from Florida

    It has the AOD with a .67 overdrive. Mostly around town driving 60-70 mph. Occasionally on the interstate to go some where but probably not very often. Would like to be able to put my foot in it every now and then when I am feeling froggy.
     
  10. The Shift Wizard
    Joined: Jan 10, 2017
    Posts: 1,958

    The Shift Wizard
    ALLIANCE MEMBER

    If the trans final ratio is overdriven it won't necessarily impact the ability to accelerate from a dead stop (or from a roll). That's more related to the first and/or low gearing ratios.
    As has been said, without more info regarding the trans gear ratios, any advice is going to be very general and not specific to your question. Both the 3.55 and the 3.73 are good choices to help your truck feel responsive on the street with the occasional stoplight joust.
    Old trucks are sort of noisy by nature so the higher, numerically, your final ratio, the more you may be missing in comfort plus the economy factor on the interstates.
     
  11. gregsmy
    Joined: Feb 11, 2011
    Posts: 46

    gregsmy
    Member
    from Florida

    I am guessing that it has the standard ratios First: 2.400:1,Second: 1.467:1,Third: 1.00:1,Overdrive: 0.667:1.
    I used one of the calculators and it looks like its only about a 100 rpm difference between 3.55 & 3.73 for the final cruising engine rpm. Anything else I can figure for its "power band" for the 0-60? Engine will be basically stock.
     
  12. The gear ratio is a straight factor in a torque calculation.

    Engine torque ? Call it 200 on this one
    200x 2.4 ( 1st gear) = 480
    480 x 3.55 = 1704
    480 x 3.73 = 1790
    480x 3.90 = 1872
    480 x 4.11 = 1968

    You might not notice 100 rpm on the highways but you’re damn sure going to notice 50 ft lbs of torque every time you take off in first gear.
     
  13. pitman
    Joined: May 14, 2006
    Posts: 4,920

    pitman

    Last roadster, began w/a vette 2:78 rear, had a Ranger Ford 5spd, overdrive 5th.
    When I changed r&p to 3:55, then it had 4 accel gears (good spread) and a level ground OD.
    The old, wide, or close ratio types matter here.
     
  14. gregsmy
    Joined: Feb 11, 2011
    Posts: 46

    gregsmy
    Member
    from Florida

    So with the original 3.27 gears it would have been 480 x 3.27= 1569 and if I can come up with a 3.73 it would be 1790. I think thats what I am going to look for 1st and if I cant, settle on the 3.55. With only about a 100 rpm difference on the top end it probably wont be very noticeable.
     
  15. Desmodromic
    Joined: Sep 25, 2010
    Posts: 571

    Desmodromic
    Member

    The difference in available torque between 3.55 and 3.73 is only about 5%, which is equivalent to varying the tire diameter by 1.5". But this may be academic in a light-in-the-back pick-up, maybe with traditional tire width; either ratio will probably spin the tires.
     
  16. Around here the majority of exploder rear ends are 3.73 limited slip.the next most common is 4:10,,, then come the lower ones.
    I like them exploder 8.8s but they are probably the heaviest “passenger” flavor rear end I’ve come across. The pinion is offset quite a bit but taking that 2-7/8” out of the long tube is pretty easy.
     
  17. Right, but if someone is already looking why not get some easy performance enhancer. That 5% anyone is going to get is the easiest low hanging fruit to grab.
     
  18. Boneyard51
    Joined: Dec 10, 2017
    Posts: 4,019

    Boneyard51
    Member

    If you have the overdrive transmission , as stated, case closed 3:73.

    You will definitely notice a difference in take off between a 3:55 vs 3:73.




    BONES
     
  19. Looks like the thing puts out 285ftlb torque @ 3000 rpm box stock. Quite peaky
    43518425-74CB-41B8-9CCF-F27713177729.jpeg
     

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