Foolproof Speedometer gear formula for dummies? TH350

Discussion in 'The "Juice Box"' started by Tom Miller, Mar 24, 2015.

  1. Tom Miller

    Tom Miller Old car enthusiast

    Does anyone have a mistake free formula to figure out and order the correct speedometer gears for TH350 trans?

    I was on TCI website, and they have a formula, but my question is this........ The whole formula is based on the number of gear teeth that you provide by counting your drive gear on the output shaft. The formula uses this number, as well as info like tire diameter,gear ratio,etc. You multiply all this info with a given number of 20168. You then take your answer and divide by 1001. The answer from this is then rounded to the nearest whole number, and that number is the gear count for the driven gear that you will want to order for your gear ratio.
    Sound clear as mud yet? Good.

    I don't see how you can run different tooth count driven gears off the same drive gear without chewing teeth off? To me it would be like trying to run the pinion gear from a 2.73 gear set with the ring gear from a 3.23 set....it ain't gonna work.

    Any auto trans guys have any insight on this?

    Thanks,
    Tom
     
  2. LARRY70GS

    LARRY70GS a.k.a. "THE WIZARD" Staff Member

    The TCI formula is correct and that is what I use. The sticking point for most guys is the number of teeth on the DRIVE GEAR. You have to count the teeth on a helical gear and it isn't easy looking through the hole for the sleeve. It is doable though. The THM350 uses clip on DRIVE GEARS that are different colors, and sometimes you can just look at the color. I'd still count just to be sure. Sometimes you HAVE to change the DRIVE gear because the resulting DRIVEN gear for a given combination just isn't available (example 46 tooth DRIVEN GEAR doesn't exist). The factory used cable speed adapters in some stock applications.

    I believe the teeth on the driven gears are all the same size so you can drive them with the same DRIVE gear. The sleeves take a range in DRIVEN gears, so you have to match the sleeve to what gear you need to run.
     
  3. Tom Miller

    Tom Miller Old car enthusiast

    That's the other problem with the TCI site.
    It doesn't specify if the gears are for the old style tailhousing that uses the small bullet, or the tailhousing with the big diameter hole????? It doesn't say if the gears will work in both tailhousings, or just one style??????
    And yes, a yellow 19 tooth drive gear that probably had a 2.28/2.56/2.73 isn't going to be worth a crap when your trying to come up with a combo for a 4.11 gear.

    To make things more clear,they should give a ballpark tooth count that you will more than likely need for a desired gear ratio. For instance, a black 8 tooth drive gear will have a better chance of finding a mating driven gear for a 4.11 rear gear, than a yellow 19 tooth drive gear.

    There needs to be something easier to clear things up for the guy at home, instead of having to use an Albert Einstein formula and HOPE it comes out right.:Smarty:
     
  4. LARRY70GS

    LARRY70GS a.k.a. "THE WIZARD" Staff Member

    Tom,
    How about giving us some numbers to chew on. What tire height, and gear ratio do you have? Do you know the tooth count of your DRIVE gear?
     
  5. gsgns4me

    gsgns4me Well-Known Member

    There is a good chart that relates the color of speedo gears, both drive and driven, with the number of teeth for the gear on page 10 of the July/ August 1998 issue of the GS X-tra. It was written by Greg Friend. It covers TH400's, TH350's, Super Turbine 300's, and TH200-4R's.

    I have a scanned copy of the page in my tool box. The quality of the copy is pretty poor or I'd post it. If I can find the original, I'll try to post it. It might be helpful to someone.
     
  6. LARRY70GS

    LARRY70GS a.k.a. "THE WIZARD" Staff Member

    My copy of that issue is kinda wrinkled but it should be readable. Not sure if it will help Tom though.

    [​IMG]
     
  7. gsgns4me

    gsgns4me Well-Known Member

    Thanks,Larry. I just thought someone watching this thread might find it useful.
     
  8. BRUCE ROE

    BRUCE ROE Well-Known Member

    Those trans gears are designed for a rather sloppy fit, so a drive gear can fit a number
    of driven gears. Any can be changed with a TH350 or TH400 in the car, with support and
    the tail piece off. You will have more options with the big gear than the bullet unit on a
    TH350; then the driven are the same for 350/400. They do wear out some day, probably
    annoy you with a leak before they stop running.

    With an extreme axle ratio, you may be outside the range of available speedo gears. So
    use an external speedo cable gearbox to shift the ratio back in range.

    All those formulas will get you in the ballpark, if you don't make any mistakes. They won't
    be as precise as I like, because real tires are FLAT where they contact the road. That means
    the running tire dia is dependent on the internal belt, which varies somewhat from one brand
    or model to another.

    If you want it precise the first try, find a calibrated road (don't count on mile markers) or use
    your GPS to determine exactly what ratio you are presently off by. Count your gear teeth, calc
    the present ratio, and multiply by the error to figure what ratio you really want. Then find a
    set of gears giving the new ratio. Bruce Roe
     
  9. philbquick

    philbquick Founders Club Member

    It's not that complected! Take a ride in an interstate, about 10 miles, and use the mile markers the calculate the percent or error on your odometer. Count the teeth on your spedo gear and increase the tooth count if your odometer reads more miles than you actually traveled by the same percentage. If your odometer reads low decrease the tooth count accordingly. After your odometer if close enough, you can adjust the speedometer by adjusting the the hair spring in the speedometer, the one that returns to needle to zero. Find a road that has a speed display or use a GPS to adjust the hair spring.
     
  10. wkillgs

    wkillgs Gold Level Contributor

    I did the match once upon a time.... works out to 1,000 revolutions per mile at the speedometer.
    1 mile = 5,280 feet= 63,360 inches
    A 27" tall tire travels (diameter x pi)= 84.78 inches in one revolution.
    Tire revolutions per mile = 63,360" / 84.78" = 747.35 revolutions.
    For a 3.08 rear ratio, multiply by the above 747.35 revolutions= 2301.82 driveshaft revolutions per mile.
    You want 1,000 revolutions per mile at the speedo cable, so divide the above 2301.82 and you get 2.3....... you need a 2.3 ratio with your speedometer gears.
    If you have 8 tooth driven gear, you need a (8 x 2.3) drive gear, or 18.4 tooth drive gear. A whole number would be more exact, such as 17 drive gear and 39 tooth driven gear...... 17 x 2.3 ratio= 39.1, which is a pretty good match.

    Just 'simple' math. (Geez, I hope I got it all right!)
     
  11. Tom Miller

    Tom Miller Old car enthusiast

    The car is in a million pieces,so driving and figuring it out isn't part of the equation. I have a calculator, and I already had the formula.
    Thanks for the input guys.

    Where is the page from a GM assy manual that says, if you have a G70-14 tire with a 4.10 gear, you need pink drive gear, blue driven gear?? I'm sure GM had something much more simple than using a GPS, or a math formula, or TCI website.

    No one answered the range of the bullets either.

    I already posted above, yellow 19 tooth drive gear, and I know it's not going to work for a 4.11
     
  12. LARRY70GS

    LARRY70GS a.k.a. "THE WIZARD" Staff Member

    The Chassis manual and Assembly manual would have that information if the car came that way from the factory.
     
  13. wkillgs

    wkillgs Gold Level Contributor

    Do you mean the holder for the driven gear?
    GM calls it a 'sleeve & seal'. Find it in group 4.338 of a parts book.

    For a TH400:
    #1362294 fits 34 to 39 tooth
    #1485026 fits 40 to 43 tooth. 73-74 is #1362293

    For a TH350:
    #1362284 fits 36 to 39 tooth
    #1362285 fits 40 to 43 tooth

    Ratio adapters are in group 4.329 if you need to go that route. New ones are expensive tho.

    There's an online parts book at 65gs.com. Look in the 'electronic garage'.
     
  14. wkillgs

    wkillgs Gold Level Contributor

    F70-14 tire = 26.8" diameter
    with 4.11 rear, you'll have 3093 driveshaft revolutions per mile..... you want 1000 revolutions at the speedo.
    You need a 3.093 ratio in your speedo gears and adapter.

    If you already have a 19 tooth drive gear and don't want to change it, you could use the 43 tooth driven gear= 2.26 ratio, and will need a ratio adapter with 0.73 ratio. There is an adapter with 0.7333 ratio, pn 1367644


    I have a chart for '66 GS.... 4.30 rear with 7.75 x 14 tires uses 17 tooth drive gear, 41 tooth driven gear and 0.7333 ratio adapter.
     
  15. BRUCE ROE

    BRUCE ROE Well-Known Member

    I also prefer to set the odometer first, then adjust the speedo to match. You will find the odometer
    gearing to be 7, 11, 13 to one, so 1001 cable turns per mile indicated. Making your measurement
    on your components and making a correction will be the most accurate. Using PI times the dia will
    give tire outer circumference. But since you really want the "rolling circumference" of the inner
    belt, speedo would read a bit high. Bruce Roe
     
  16. Scott King

    Scott King Well-Known Member

  17. Jeff Mann

    Jeff Mann Well-Known Member

    Hey Tom,

    Not sure if you got this figured out already, but if not this page written by my late uncle might provide some helpful info:

    http://chevelleengineer.home.comcast.net/~chevelleengineer/speedo.htm

    The tables in the middle of the page are kinda jacked-up but I think you can still make out the BOP TH350 speedo gear part numbers. Uncle Fred was an engineer at Oldsmobile and drag racer during the Glory Days. Later in life he got into trucks and his last musclecars were a red 1970 LS-5 4-speed El Camino and a GMC Syclone.

    Jeff
     

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