Register now to get rid of these ads!

Technical Real world driving...Flathead with a 3 speed

Discussion in 'The Hokey Ass Message Board' started by nick28, Nov 17, 2014.

  1. nick28
    Joined: Jun 5, 2009
    Posts: 377

    nick28
    Member
    from Kentucky

    Hey guys,

    Just picked up a '39 Deluxe tudor sedan with original trans, 3.78 rearend, and '47 Flathead.

    I really want to keep the flathead and I am considering keeping the 3spd trans as well as long as I can drive the thing at a reasonable speed.

    Rear tire will most likely end up a 7.00, possibly a 7.50.

    Will I be able to cruise this thing 60-70 mph comfortably? I read some limited info here that with that gear and tire size it should be just right.

    Thanks
    Nick
     
  2. I had a '32 pickup with the same combo. It felt more "natural" at 60 than 70. A 3;54 gear and you would probably like it a lot more. Tim
     
    Unibodyguy likes this.
  3. Jimmy2car
    Joined: Nov 26, 2003
    Posts: 1,707

    Jimmy2car
    Member
    from No. Cal

    Tim is correct, but with a 7.50 rear tire at 65 mph, your engine will be right at 2800 rpm. Not too bad at all. 3.54 would be a bit better, tho
     
    Unibodyguy likes this.
  4. seldom scene
    Joined: Oct 9, 2002
    Posts: 867

    seldom scene
    Member

     

  5. seldom scene
    Joined: Oct 9, 2002
    Posts: 867

    seldom scene
    Member

    Bear in mind that this car was built when the national speed limit was 50 mph, and most roads were dirt. It was not intended to be a freeway flyer nor a on a dime stopper. That's why we HotRod them.
     
    Hnstray likes this.
  6. Kiwi 4d
    Joined: Sep 16, 2006
    Posts: 3,564

    Kiwi 4d
    ALLIANCE MEMBER

    With 3.78. Gears and 750x16 tyres you should pull 2677rpm at 60mph
    With 3.54 gears and 750x16 tyres you should pull 2507 rpm at 60 mph
    Formula desired speed (60mph)x rear end ratio(3.78) x366 (a constant) divided by tyre height (750 x16 =31") =2677 rpm.
     
    Unibodyguy likes this.
  7. mlagusis
    Joined: Oct 11, 2009
    Posts: 1,127

    mlagusis
    Member

    Is there a 3 speed OD that will bolt in? That would be the way to go.
     
  8. 57Custom300
    Joined: Aug 21, 2009
    Posts: 1,424

    57Custom300
    Member
    from Arizona

    That's what I was thinking. My 38 (mechanical brakes) will run 60+ but you better have a clear 40 acres to stop it.
     
  9. tubman
    Joined: May 16, 2007
    Posts: 6,916

    tubman
    ALLIANCE MEMBER

    I have a '51 with a stock drivetrain (no overdrive) with 3.55 gears. I drove it from the twin cities to Chicago for a car show about 20 years ago. I was in a hurry to get home, so I took the interstate back and had no problem keeping up with traffic. I agree with the guys recommending 3.54's.
     
    Unibodyguy likes this.
  10. 4dFord/SC
    Joined: Sep 12, 2004
    Posts: 837

    4dFord/SC
    Member

    Mitchell Manufacturing makes a bolt in unit that gets high marks from users: 1-800-859-2088. Another option is a Columbia Two-Speed Axle from back in the day. They're still out there, but a little pricey, and finding all the controls can be a challenge. Mine with 3.78 rear end will happily cruise all day at 70+.
     
    Unibodyguy likes this.
  11. 36cab
    Joined: Dec 2, 2008
    Posts: 896

    36cab
    ALLIANCE MEMBER

    Wasn't there a thread on Ford Barn about grafting a Volvo OD unit in to a Model A torque tube? Would something like this work for a V8?
     
  12. Petejoe
    Joined: Nov 27, 2002
    Posts: 12,264

    Petejoe
    Member
    from Zoar, Ohio

    If you plan on adding an overdrive you could also consider a T-5.
    My 37 is running an 8BA with the original 3 speed and highway gears. Its happy all day at 65 mph. Ran a 1000 mile trip last year in two days. Why would you need more speed for the highway in an old ford?
     
  13. nick28
    Joined: Jun 5, 2009
    Posts: 377

    nick28
    Member
    from Kentucky

    Thanks for the replies guys. I just want to be able to cruise comfortably at highway speeds when needed, without beating the engine. I have considered the t5 conversion and the overdrive is a possibility. The car is quite low and I would be a bit concerned about clearance for the overdrive though.

    Too be honest if she'll run 65 or so as is (which is sounds like it will) I prefer not to pull the drivetrain apart.
     
  14. alchemy
    Joined: Sep 27, 2002
    Posts: 20,407

    alchemy
    Member

    I've been driving a '32 tudor with a flathead and '39 trans for eight years now. It has 3.78 rear gears and 7.00x16 rear tires. This engine really purrs at 2600 RPM, but it will cruise down the road just fine at 2700-2800 to keep up with interstate traffic. I did buy a 3.54 gear set a couple years ago that I will put in some day. It seems to have enough low end torque that I don't think it will suffer around town when I put the higher gears in.
     
    nick28 likes this.
  15. Bruce Lancaster
    Joined: Oct 9, 2001
    Posts: 21,681

    Bruce Lancaster
    Member Emeritus

    My '48 has always had 3.78 and the standard 600-16's. In college, I put on lots of miles covering my known universe (New Orleans to Vt-Canada border area) and documented the car's cruising zone with stacks of 75-85 MPH speeding tickets (Outta the car, long hair!!) long before the 55 limit days. Cruising in the '60's was not a strain on the combo.
    Once, a cop about to fill in my ticket asked "Will this thing GO that fast?" I proudly replied that it could touch 100, but that wasn't the right answer apparently...
    Speedo was quite accurate at highway speeds, according to officer radar and his many friends, but I doubt the 100 area was accurate...
     
    nick28 likes this.
  16. nick28
    Joined: Jun 5, 2009
    Posts: 377

    nick28
    Member
    from Kentucky

    Sounds like she outta cruise along just fine. Thanks for the replies!
     
  17. Our 51 Merc runs a flattie and a stock 3 speed. Swapped in a 62 Econline rear with its stock gear I think the gear is in the high 2's.
    It will run 75mph all day long and pull 14 inches of vacumm doing it

    Ave mpg will be about 16-17mpg with 2 Holley 94's running syncronized linkage.
     

Share This Page

Register now to get rid of these ads!

Archive

Copyright © 1995-2021 The Jalopy Journal: Steal our stuff, we'll kick your teeth in. Terms of Service. Privacy Policy.

Atomic Industry
Forum software by XenForo™ ©2010-2014 XenForo Ltd.