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Customs Running 2.50 9inch gears.

Discussion in 'The Hokey Ass Message Board' started by DrewModelA, Aug 30, 2019.

  1. Here you go. This one has a little bit of everything.
    http://wallaceracing.com/Calculators.htm
     
  2. Mimilan
    Joined: Jun 13, 2019
    Posts: 189

    Mimilan
    Member

    I would do what you're suggesting without giving it a 2nd thought. I don't mind listening to screaming engines at the track but not on a long drive home.

    The formula for this is: MPH x 336 x Overall ratio . Then divided by [÷]Tyre diameter.
    So at 70 mph with 2.5:1 Ratio and 26.5" Tyres it would be
    [70 x 336 x 2.5] ÷ 26.5 = 2218 RPM

    This would be with a lock-up converter or manual trans.
    If you had about 5% torque converter slippage the RPM's would be about 2329 RPM

    If you shifted at 3000 rpm from 1st into 2nd, this would be at 39.4 mph with the 2.5:1 gears
    And this would be at 35.9 mph with the lower 2.75:1 gears [a difference of only 3.5 mph, so getting off the line issues are diminished by the 1st gear ratio ]

    You would need a low stall converter and a good transmission cooler
    and preferably a cam with less than 215 deg @ 050 Duration.

    As long as your engine has enough power at 2300 rpm to push that car through the air.
    It would make a pleasant cruiser

    My off topic C7 Corvette cruises at 1575 RPM at 70 mph [ EFI, variable cam timing and OD help here]

    Edit: I just did a comparison with a 57 Chevy with a powerglide and factory 3.36 rear end [and 1.82 low gear]
    The overall ratio was 6.11:1 off the line [with a 195 hp 283]
    Your Ford has a 2.4:1 first gear x 2.5 rear = 6.00:1 overall ratio in first.
     
    Last edited: Aug 30, 2019
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  3. mgtstumpy
    Joined: Jul 20, 2006
    Posts: 7,574

    mgtstumpy
    Member

    Ideally your actual engine RPM @ 60mph should be somewhere between 1800rpm and 2100rpm. Too low RPMs @ 60mph will produce vibration which is cancelled out at above RPMs. You don't want your ride to labour due to too low RPMs.
     
  4. town sedan
    Joined: Aug 18, 2011
    Posts: 1,077

    town sedan
    ALLIANCE MEMBER

    What Mimilan said! This would be the perfect place for an RV camshaft. Torque is your friend, build for it as best you can.
    -Dave
     
  5. RmK57
    Joined: Dec 31, 2008
    Posts: 1,030

    RmK57
    Member

    I wouldn't want to have gearing like that in the streets of San Francisco. Poor old cruise-o-matic would catch on fire!:eek:
     
    Old wolf and town sedan like this.
  6. sunbeam
    Joined: Oct 22, 2010
    Posts: 4,276

    sunbeam
    Member

    Without a lock up converter you don't want to get the RPM in the stall speed range or it will generate a lot of heat.
     
  7. liliysdad
    Joined: Apr 1, 2013
    Posts: 30

    liliysdad
    Member
    from Oklahoma

    Does no one like burnouts, or stop light quickies? The very idea of sub 3:1 gears makes me sad.
     
  8. DrewModelA
    Joined: Oct 26, 2014
    Posts: 36

    DrewModelA
    Member

    If I was building a car for burnouts and holeshots I wouldn’t have done a ‘60 thunderbird


    Sent from my iPhone using The H.A.M.B. mobile app
     
    Hnstray likes this.
  9. liliysdad
    Joined: Apr 1, 2013
    Posts: 30

    liliysdad
    Member
    from Oklahoma

    Just because you didnt build the car for it, doesn't mean its not fun to do every once in a while.
     
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  10. sunbeam
    Joined: Oct 22, 2010
    Posts: 4,276

    sunbeam
    Member

    The car is only a couple of hundred pounds heaver than a big block GM mid size. It is unit body Curb weight around 3800 lbs
     
    Old wolf likes this.
  11. DrewModelA
    Joined: Oct 26, 2014
    Posts: 36

    DrewModelA
    Member

    I’ve had plenty of Model A’s 2000lbs or lighter with a HOT engine and a 4speed. I’m just over it, that’s why I built a custom, I couldn’t care less if it was “hot rod” fast.


    Sent from my iPhone using The H.A.M.B. mobile app
     
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  12. liliysdad
    Joined: Apr 1, 2013
    Posts: 30

    liliysdad
    Member
    from Oklahoma

    That just doesn't sound fun at all...
     
    Old wolf likes this.
  13. Mr48chev
    Joined: Dec 28, 2007
    Posts: 25,536

    Mr48chev
    ALLIANCE MEMBER

    Here is a simple no math rpm, tire size, speed , gear ratio calculator. plug in 3 and get the 4th.
    There are a lot of them on the net but this is the simplest. http://www.advanced-ev.com/Calculators/TireSize/
    If you have an overdrive trans you need to multiply the given rpm by the % of the od to get the final rpm That is the only math involved outside of figuring out the tire diameter.
    It says that with the 2.5 gear and 26.7 inch tall at 70 the engine is running at 2202 rpm.
    with 2.75 gears it would be 2422.
    With 3.0 gears it would be 2643
     
    DrewModelA likes this.
  14. I had a 73 cutlass, all stock except dual exhaust, shift kit and flipped breather lid, with a 350/350 and 2.56 gears.
    Hard to believe but it would do great one wheel peal burnouts, it would get a second gear scratch at 60 mph shifting manually and it would shift into high by itself under full throttle at 103 mph.
    I think you'll be just fine with your choice.
     
    DrewModelA likes this.
  15. s55mercury66
    Joined: Jul 6, 2009
    Posts: 3,677

    s55mercury66
    Member
    from SW Wyoming

    I would like to have a set of those in my 4200 lb Mercury, but it has a 428. I'm in the try it camp, it's pretty easy to change the center section, and put in something lower if you don't like it.
     
    DrewModelA likes this.
  16. GearheadsQCE
    Joined: Mar 23, 2011
    Posts: 2,145

    GearheadsQCE
    ALLIANCE MEMBER

    I'v run 2.42 gears in the Quickchange in my OT Crew Cab shop Truck. Over 5000 lbs.

    Do it! You'll be fine.
     
    DrewModelA likes this.

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