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Technical 350 TRANS GEARS

Discussion in 'The Hokey Ass Message Board' started by nailheadjay, Oct 3, 2016.

  1. nailheadjay
    Joined: Jul 22, 2012
    Posts: 102

    nailheadjay
    Member

    NEED A SPEEDOMETER GEAR, THEY OFFER DIFFERENT TEETH, HOW DO YA KNOW WHICH ONE YA NEED. TRANS AND MOTOR ARE NOT FROM THE 51 BUICK. THANKS
     
  2. Mr48chev
    Joined: Dec 28, 2007
    Posts: 27,327

    Mr48chev
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    If you have a gps that reads road speed or a smart phone that you can download a speedometer app on
    [​IMG]

    You can run down the road at say and even 60 mph on either the gps or speedo and note what the other one reads.
    Then when you get back to the shop you figure out what percentage the speedo is off and in what direction.
    Then knowing what the teeth on your drive and driven gears are you go to the chart and figure out what combo will give you the correct reading. The only hard part is pulling the driveshaft and extension housing.
    you can probably calculate it out by rear end gear ratio and tire size and come out close but I have never yet tried to do it that way before actually driving the rig to see if and how much it is off.
    Here is a page that tells you exactly how to do that before you put it together.
    http://www.tciauto.com/tc/speedometer-gear-calculator/
     
  3. squirrel
    Joined: Sep 23, 2004
    Posts: 46,694

    squirrel
    Member

    like I said in your other post, it depends on rear gear ratio and tire size. Tell us the rear gear ratio and the tire size. Or look it up online.

    http://www.tciauto.com/tc/speedometer-gear-calculator/

    you need to have two gears that work together to give you the ratio you need. See if you can figure it out from the web page...if not, ask specific questions and we should be able to help you figure it out.

    does the tail housing have a large or small hole in it? Chevy TH350s used a small hole, Buick-Olds-Pontiac had a large hole. The gears and sleeve are different. Small is less than 1 inch diameter, large is over 2 inches diameter.
     
  4. orangepeel468
    Joined: Jun 8, 2017
    Posts: 7

    orangepeel468

    how about real speed is 33 mph and the speedo is reading 70. turbo 350 trans. did the chart thing and this is where I'm at. any help would be great thanks, tony
     
    Last edited: Jun 8, 2017

  5. you will need to know the rear axle ratio, rear tire diameter AND what the drive gear is inside the tailshaft of the transmission...plug them into the calculator and you know what driven gear . just a guess, but if it's that far off you may need to change the drive gear too

    simple...even a child can do it
     
  6. squirrel
    Joined: Sep 23, 2004
    Posts: 46,694

    squirrel
    Member

    If the speedo is reading 70 when the real speed is 33, then something is seriously wrong. First thing I would do is to check the odometer reading against a measured mile, to see if the speedometer gears are close to correct, or are way off. If the speedometer is worn out, it could be that the odometer is close, but the speedo is way off.

    Do that, let us know
     
  7. squirrel
    Joined: Sep 23, 2004
    Posts: 46,694

    squirrel
    Member

    A speedo drive gear for a BOP, used with a driven gear for a Chevy, might do that, too. Then the odometer would be way off.
     
  8. 283john
    Joined: Nov 17, 2008
    Posts: 787

    283john
    Member

    You're not doing a perpetual burnout everywhere you go, by chance?
     
  9. gimpyshotrods
    Joined: May 20, 2009
    Posts: 17,480

    gimpyshotrods
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    If it really is correctly reading 70 at 33, you need a 50% reduction box, and then correct the rest through the drive/driven gears.

    You can get a 50% reduction box from: https://transmissioncenter.net/shop/gm-dodge-ford-ratio-adapter/

    They are $88.* That would get the speedometer to to 35 at 33. From there, you should be a driven gear change away from correct (if that is not close enough).

    Although I suspect a busted speedometer. Check, as suggested, against a measured mile of GPS phone speedometer app.

    *They are running a 10% off sale right now, too. FYI.
     
  10. squirrel
    Joined: Sep 23, 2004
    Posts: 46,694

    squirrel
    Member

    there should be no need to use a 50% reduction box...unless you're dead set against changing the gears in the transmission.

    Then again, I guess most folks would rather spend money, than fix the cause of a problem.
     
  11. gimpyshotrods
    Joined: May 20, 2009
    Posts: 17,480

    gimpyshotrods
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    I have never seen a speedometer cable output at >200% the necessary RPM.

    Most GM stuff that I have worked with expect an output of 1000 revolutions per mile, regardless of GM division.

    There is either a severe mismatch between the transmission brand/age and the speedometer head brand/age, in terms of the expected revolutions per mile. If the delta really is that big, then there might not be a way to correct it with any combination of drive/driven gears.

    If the TH350 is all stock, it should be close to 1000 revolutions per mile output, give-or-take rear ratio or tire height change. Unless he has cut his tire circumference dramatically, or made a radical change in rear gear ratio, or both, it is hard to comprehend a >2X change.

    A TH350 does not have a eccentric, re-position-able speedometer drive, that can accommodate a huge range of driven gear diameters like other transmissions do.

    It would be good to know what the speedometer head is from, and have it confirmed that the speedometer head is not broken.
     
    Last edited: Jun 8, 2017
  12. gimpyshotrods
    Joined: May 20, 2009
    Posts: 17,480

    gimpyshotrods
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    Oh, and several models of GM vehicles used in-line ratio correctors, right off of the assembly line.

    I have a stack of them from S10 T5's that I pulled off before installing them in other vehicles.
     
  13. orangepeel468
    Joined: Jun 8, 2017
    Posts: 7

    orangepeel468

    Its in a 1947 Plymouth. the speedo has been rebuilt. did the 3.08 gears with a
    1947 Plymouth coupe 350/350 3.08 gears and 235/70/15 tires.27.95 speedo was restored. has a 22 driven gear and a 10 drive gear. I will do the mile thing as soon as it stops pouring down rain. thanks tony.
     
  14. gimpyshotrods
    Joined: May 20, 2009
    Posts: 17,480

    gimpyshotrods
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    I will see if I can dig up what that speedometer is indexed to, in terms or revolutions per mile. In the mean time, look at the face, and see of there are any tiny numbers written on it. Some brands have the number on the face. It is not super common in OEM stuff, though.

    Cluster looks like this?
    [​IMG]
     
  15. squirrel
    Joined: Sep 23, 2004
    Posts: 46,694

    squirrel
    Member

    since you have the correct gears in the transmission, it must be something with the speedo....

    It's an original Plymouth speedometer, right? I think they use the normal 1000 turns per mile standard.
     
  16. squirrel
    Joined: Sep 23, 2004
    Posts: 46,694

    squirrel
    Member

    GM did indeed use reduction boxes, I have a few old ones that I've used for various purposes...they're pretty handy if you want to swap rear gears for a short time, just add the box at the same time you do the gear swap, take it out when you go back to highway gears.

    With a transmission set up for 2.29 rearend gears, swap in some 4.56 gears in the rearend, it will be off that far
     
  17. orangepeel468
    Joined: Jun 8, 2017
    Posts: 7

    orangepeel468

    there is a tail light bucket for your falcon on ebay.
     
  18. orangepeel468
    Joined: Jun 8, 2017
    Posts: 7

    orangepeel468

    yes it is. owner told me it has been restored. he said he spent 1000.00 for it. must be nice to have deep pockets.
     
  19. orangepeel468
    Joined: Jun 8, 2017
    Posts: 7

    orangepeel468

    yes it does.
     
  20. gimpyshotrods
    Joined: May 20, 2009
    Posts: 17,480

    gimpyshotrods
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    I'm not seeing it. All I see is a guy who is selling black and white pictures of them.

    I bought a beat-up pair, so I can at-least recover from the accident damage, and find a better one.
     
  21. gimpyshotrods
    Joined: May 20, 2009
    Posts: 17,480

    gimpyshotrods
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    3.08 gears and 235/70/15 tires, 27.95, 22 driven gear and a 10 drive gear.

    I am showing my math, in case I made a mistake. Someone will spot it.

    Tire diameter (aprox): 27.95"
    Pi (rounded): 3.1416
    Tire circumfrence (aprox): 87.82"
    Tire revolutions per mile: 721.51
    Driveshaft revolutions per mile: 2222.24
    Drive:driven ratio: 10:22
    Ratio converted to fraction: 0.46
    Speedometer cable revolutions per mile with given gearset: 1022.23

    As far as I can tell, the Plymouth speedometer wants to see 1000 revolutions per mile. Your speedometer cable should be turning at 1022.23. As no two tire manufacturers make perfectly sized tries, and they even differ from from model-to-model in the same brand, and certainly vary with wear, the 1022.23 revolutions per mile could me considered within the margin-of-error of dead-on. That's closer than my wife's VW.

    The speedometer is either broken, or is not a 1000 revolutions per mile unit.
     
  22. gimpyshotrods
    Joined: May 20, 2009
    Posts: 17,480

    gimpyshotrods
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    If it is another speed other than 1000, it should be easy enough to tell. Just check it against a measured mile.

    If the odometer clocks ~2.12 miles over the measured mile, then the speedometer and odometer are reasonably in agreement. Adjust via whatever strategy you find most appropriate.

    The odometer is a direct gear drive. When the cable turns, it turns, via gears. It either works, or it does not.

    The speedometer is not directly driven. It has a magnet that moves a cup, indirectly, and that rotating cup winds springs, and those move the needle. When those springs weaken, the needle moves more easily, and gives a higher reading.
    [​IMG]
    The speedometer might have been "restored" but only cosmetically.

    Many metro areas, although they are disappearing, have speedometer shops that can actually restore the mechanical functions.
     
    Last edited: Jun 8, 2017
  23. squirrel
    Joined: Sep 23, 2004
    Posts: 46,694

    squirrel
    Member

    there's a fellow on here who does them, also. I haven't had him do any of mine yet, but I probably will one of these days.

    thanks for the explanation of how they work....it mostly appears to be magic, but it turns out it's a pretty simple thing. The usual problem area is the bearing between the magnet and cup. And sometimes the magnet and cup have their magnetic properties get "messed up" some how.
     
  24. orangepeel468
    Joined: Jun 8, 2017
    Posts: 7

    orangepeel468

    thanks for all the info. learn something new every day. great site, great help from everybody. did the measured mile, comes out 2 for 1. think I'm going to do the 50% reduction gear. should put me right where I need to be. Thanks Tony.
     
  25. gimpyshotrods
    Joined: May 20, 2009
    Posts: 17,480

    gimpyshotrods
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    I think that might be the only way to go.

    The highest tooth count drive gear I have seen has 10-teeth. You already have that one.

    The highest tooth count driven gear I have seen has 22-teeth. You already have that one.

    The lowest tooth count drive gear that I have seen has 7-teeth.

    The lowest tooth count driven gear that I have seen has 17-teeth.

    Your current drive driven decimal ratio is 1:0.4545. That is giving you about 1000 revolutions per mile, at the cable.

    If you kept the 22-tooth driven gear, and put in a 7-tooth drive gear, you'd have a 1:0.3182 ratio. That would give you 707.08 revolutions per mile. Better, but not fixed.

    The 50% reduction box will but the cable at 511.115 revolutions per mile. Showing 70 at 33 will become 35 at 33. That's a few percent off still, but pretty close. That then could be corrected with drive/driven gears.

    At least at 70, it would be ~4mph over, rather than under. Still closer than my wife's VW.

    As tire heights, and thus circumferences vary quite a bit, you should observe your exact speed with a GPS or GPS phone app, and make any corrections based on actual equipment and operating conditions.
     
    Last edited: Jun 9, 2017
  26. squirrel
    Joined: Sep 23, 2004
    Posts: 46,694

    squirrel
    Member

    you've never seen a BOP TH350? :)
     
  27. gimpyshotrods
    Joined: May 20, 2009
    Posts: 17,480

    gimpyshotrods
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    I have, and the OP does not have one.

    If you have a link to all of the parts necessary for him to correct this, with h BOP parts, please post it.
     
  28. squirrel
    Joined: Sep 23, 2004
    Posts: 46,694

    squirrel
    Member

    I doubt it would work...and it would be more trouble than getting a reducing box set up.

    Keep the 10 tooth drive gear, put a BOP tailhousing on it, with a 44 tooth driven gear. They use a 700R4 style sleeve.
     
    gimpyshotrods likes this.
  29. orangepeel468
    Joined: Jun 8, 2017
    Posts: 7

    orangepeel468

    I might have that in the back. I will give it a shot.
     
  30. squirrel
    Joined: Sep 23, 2004
    Posts: 46,694

    squirrel
    Member

    Check the angle of the gear teeth, and the gear diameters, they might be quite different, since the 10 tooth gear is not designed to mesh with the 44 tooth gear
     

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