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Rear axle swap gear ratio help

Discussion in 'The Hokey Ass Message Board' started by 50coupe, Jul 14, 2011.

  1. 50coupe
    Joined: Mar 23, 2005
    Posts: 46

    50coupe
    Member
    from Pittsburgh

    I have a 54 chevy with the stock motor transmission and rear end. I am planning on keeping the motor, but putting in a T5 and new rear end. I already have the trans and bellhousing, but I need help on figuring out what rear gears I need to run. I am switching these things out to be able to cruise a little better on the highway. I tried searching here, but I cant seem to find the right info yet. I know I need a 60" wide rear end (mounting surface to mounting surface), and I have an idea of some of the cars that have that width, I am just unsure of what ratio I should be looking for. Any and all help would be greatly appreciated.
    Thanks,
    Matt
     
  2. wuuups... I was speed readn'

    you need this instead
    http://www.corral.net/tech/gearcalc.html

    Figure out which T5 you have as they came in a couple different final drive ratios.

    Then figure about 2200-2800 rpm range which will give the best mileage. (I like 2300-2400)

    Punch those figures in and your desired cruising speed
    (I use 70-75)
     
  3. drptop70ss
    Joined: May 31, 2010
    Posts: 1,186

    drptop70ss
    Member
    from NY

    what is the first and fifth gear ratios of the T5? Height of the rear tires? That will help determine what rear gear to run. Dont want the engine to lug in first or first to be too short, and want 60mph to be around 2000 RPM for easy highway cruising.
     

  4. 50coupe
    Joined: Mar 23, 2005
    Posts: 46

    50coupe
    Member
    from Pittsburgh

    I think all the t5's shown on that dropdown menu are ford, I have a chevy t5.
     
  5. 50coupe
    Joined: Mar 23, 2005
    Posts: 46

    50coupe
    Member
    from Pittsburgh

    My 1st gear is 4.03:1 and I'm not sure of my 5th. It's either 0.72 or 0.76 or 0.86. My tires are 195/75/14
     
    Last edited: Jul 14, 2011
  6. drptop70ss
    Joined: May 31, 2010
    Posts: 1,186

    drptop70ss
    Member
    from NY

    :eek: 4:03 is way low, and the tires are also short. No need for steep gears with that combo, a 2:73 to 3:00 would work, but first gear is going to be short no matter what gear you run because it is so steep. Almost seems like a granny gear, is it a truck transmission? What ratio is 2nd? I have a 3:36 first with a 3:08 in one of my T56 cars and it is fine, but if I had your 4:03 I would be in first gear for about a second.
     
  7. 50coupe
    Joined: Mar 23, 2005
    Posts: 46

    50coupe
    Member
    from Pittsburgh

    I think this is what I have, it's out of an s10. 1st – 4.03 2nd – 2.37 3rd – 1.49 4th – 1.00 5th – 0.86
     
  8. drptop70ss
    Joined: May 31, 2010
    Posts: 1,186

    drptop70ss
    Member
    from NY

    with that short of a first and that overdrive, I would run around a 2:73 gear. Will be around 2000 RPM at 65 MPH and you should be able to use first gear.
     
  9. F&J
    Joined: Apr 5, 2007
    Posts: 13,221

    F&J
    Member



    I see where you are going with that first gear ratio, but I don't think that stock 54 motor can handle 5th gear with 2.73:confused:



    I am basing this on a later 250 six with a 3.73. It needed planning ahead on the highway if you saw a long grade coming up. Otherwise it would run out of steam, and have to be downshifted.
     
  10. I always rekon with a manual around 3.0---3.25 is a good range for cruising an economy.
     
  11. F&J
    Joined: Apr 5, 2007
    Posts: 13,221

    F&J
    Member


    With an very small motor and an .86 OD?
     
  12. I have an older 2WD S10 Blazer with the 4cyl and a T5. Came with 4:11 gears from the factory. Works well with the anemic 95HP motor and there's always a gear to choose from.

    It turns 2600 @ 75mph with a 275/60/15 rear tire and gets 29-30 mpg on the highway. The plus side is, nobody can beat me out of the hole at a stoplight. (then I have to shift)

    With the 4:11 gears, 1st gear is nearly a granny gear.

    -------------------------
    for you

    2200 rpm
    75 mph
    .71 fifth gear
    195/75/15 tire

    =3.25905854862089 rear gear
     
  13. Hnstray
    Joined: Aug 23, 2009
    Posts: 12,306

    Hnstray
    ALLIANCE MEMBER
    from Quincy, IL



    I agree that a 2.73 seems too tall for an OD, even at .86. as that equates to about 2.35. For a stock '54 Chev 6 that is pretty tall. I realize tire diameter is a factor here, but a 195/75-15 is going to pretty close to the orignial 6.70-15.

    The 3.25 will effectively be about 2.80 with OD. With the 6 cyl in question I would think that is about as high (numerically low) that would work satisfactorily in general highway use.

    Ray
     
  14. fiftyv8
    Joined: Mar 11, 2007
    Posts: 5,281

    fiftyv8
    Member
    from CO & WA

    That is the trick balancing 1st and 5th gears with your tires and diff ratio and your engines ability to keep things ticking along.
    There have been a few old posts which raised the issue of running too lower engine rpm's in O/D.
     
  15. shoprat
    Joined: Dec 23, 2006
    Posts: 1,109

    shoprat
    Member Emeritus
    from Orange, CA

    I would use a 3:70 gear
     
  16. Hnstray
    Joined: Aug 23, 2009
    Posts: 12,306

    Hnstray
    ALLIANCE MEMBER
    from Quincy, IL


    With the tire diameter indicated, that would be the farthest I would go unless the rear tire size was increased in diameter.

    A for 1st gear being too low, yes, it is low, but so what. Start in second if you want. At 2.37 plus the 3.70 rear end, it should get off the line ok for normal driving and you still end up with the cruising gear you want in 5th.

    The MOST ideal solution would be a different set of T-5 gears. If you go www.BritishV8.org, they have an excellent listing of all the T-5 applications and their gearset ratios. The only thing really desireable about your S-10 T-5 is the tailhousing that has the forward mounting position for the shifter. It can be fitted to any other T-5 rather simply.

    Ray
     
  17. drptop70ss
    Joined: May 31, 2010
    Posts: 1,186

    drptop70ss
    Member
    from NY

    guys he said his tire is a 195 75 14, not a 15, very short tire. I honestly dont have any experience with that early a straight 6, but most straight engines do make decent torque. I normally try and get to 2000-2200 RPM at 60 MPH, keeps things reasonable so you can run at 75 MPH if you want to and still not have to wind up the engine. My T56 has a .080 5th gear, and I run around 2000 RPM at 60. I dont use 6th until 70MPH or better. This is in my 65 olds, 3:08 rear gear. I think he should run a taller tire and use more rear gear, similar to what the factory had with that trans..otherwise run a higher gear to offset the shorter tire.
     
    Last edited: Jul 14, 2011
  18. 50coupe
    Joined: Mar 23, 2005
    Posts: 46

    50coupe
    Member
    from Pittsburgh

    Thanks for all the input I feel like I have a better idea of what is going on here. I'm still not decided, but I see the different directions I might be able to go.
     
  19. R Pope
    Joined: Jan 23, 2006
    Posts: 3,309

    R Pope
    Member

    I ran a '55 4.11 rear and 3 speed OD in my '54 Bel Aire. It would go faster in High than it would in OD! With 15" tires it spun 3000 RPM at 60mph in High, and 2100 in OD. The 235 didn't appreciate that tall gear at all. It was about like a set of 2.90 gears, too tall for that car and engine combo.
     

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