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Flattie / T-5 Rear end Ratio?

Discussion in 'The Hokey Ass Message Board' started by 87Heaven, May 9, 2011.

  1. 87Heaven
    Joined: Dec 20, 2009
    Posts: 71

    87Heaven
    Member

    I am at that point in my project where I have to decide what rear end ratio to use.
    It's a 1940 Ford Coupe about 2800 lbs, pretty well modified 276 ci 51 Merc
    a T-5 w/ .74 overdrive ( I forgot the 1st gear ratio) I think it 3.50.
    I have a set of 3.07 and a 3.73 gear set on hand.
    Which one should I use.
    I ran the calcs on both so technicaly I'm aware of Speed vrs RPM.
    But in reality I would like to hear from someone first hand who's running this set up.
    There is allways more to it than just running the numbers.
    Thanks in advance.
    Manuel
     
  2. Dirty Dug
    Joined: Jan 11, 2003
    Posts: 3,687

    Dirty Dug
    Member

    I just got my '30 roadster on the road. It has a 255 flathead, t-5, and an eight inch. I'm running 3.40 gears with 7.50x16s. It turns 1900 rpms at 70 mph, with good pick up in first gear. You're probably not running that big a rear tire so I'd say the 3.73 would work out quite well for you. hope that helps.
     
  3. Deuces
    Joined: Nov 3, 2009
    Posts: 22,024

    Deuces
    Member
    from Michigan

    First gear ratio in a Ford V-8 T-5 is 3.35 I think??
     
  4. flatheadpete
    Joined: Oct 29, 2003
    Posts: 10,241

    flatheadpete
    Member
    from Burton, MI

    I had 4.10 gears in my banjo with a Merc and t5. Got over 30 mpg and ran like all hell.
     

  5. nailheadroadster
    Joined: Jun 7, 2006
    Posts: 1,522

    nailheadroadster
    Member

    The rear tire diameter will make all the difference. Like Dirty Dug said, a 31" diameter tire (like his) versus a... I dunno, a 28" tire will change everything. Try the numbers using this :

    http://www.csgnetwork.com/multirpmcalc.html
     
  6. fiveohnick2932
    Joined: Mar 29, 2006
    Posts: 901

    fiveohnick2932
    Member
    from Napa, Ca.

    Need to know the tire Dia. Without knowing how your Flattie performs you could
    go with 3.73

    Im putting 3.73 in my coupe with a flathead and 31" tires. Should really fly down the hwy! May even need to go to 3.90s
     
    Last edited: May 9, 2011
  7. Ole don
    Joined: Dec 16, 2005
    Posts: 2,915

    Ole don
    Member

    With that much weight and barn door effect, run the 3.73
     
  8. 87Heaven
    Joined: Dec 20, 2009
    Posts: 71

    87Heaven
    Member

    You guys are awsome.
    I feel better. I was leaning towards the 3.73 but I was affraid of having a stump puller where first gear was good for about 5 feet:)

    I havent decided on tire size, but it wont be too big. The car will be lowered some and I just want to fill the wheel well. What I have on there now is about 26" tall. The tire size can change as time goes on.

    Thank again guys!
     

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