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TH350 Speedo Gear Advice needed

Discussion in 'The Hokey Ass Message Board' started by eldojoe, Apr 30, 2012.

  1. eldojoe
    Joined: Oct 27, 2010
    Posts: 3

    eldojoe
    Member
    from Wisconsin

    My Speedometer is reading way lower than actual. I could really use some advice on changing the drive gear at the transmission before I pick up some tickets and all the BS that comes with it. Indicated speed is about 82 % of actual, for example the speedo reads 45 MPH, but I'm really doing 55. If my speedo says 65 MPH, it's really closer to 84. This could get expensive! As far as I can tell, It appears to be a stock '72 Nova TH 350. The whole drive train may be based on that, but I'm running 27 inch diameter rear wheels/ tires. The axle ratio is unknown. I've got a 22 tooth drive gear.
    Do I need more or less teeth on the new gear? Any suggestions? What options are availible? Thanks in advance....Joe
     
  2. Lobucrod
    Joined: Mar 22, 2006
    Posts: 4,122

    Lobucrod
    Alliance Vendor
    from Texas

    You will need a driven gear (the one that the speedo cable attaches to) with less teeth. This will speed up the speedo. If your 20% off then you will need a gear with 20% less teeth. You will be limited to how much fewer teeth are available on the gears. If you can't put one in small enough then you will need to get a drive gear (the one on the output shaft) with more teeth.
     
  3. Dane
    Joined: May 6, 2010
    Posts: 1,351

    Dane
    Member
    from Soquel, CA

  4. Aquaroscoe
    Joined: Apr 13, 2006
    Posts: 315

    Aquaroscoe
    Member

    Easy to change out, I suggest you take out the driven gear (speedo cable connection one) and shine a flashlight in there to look for color (both drive and driven) You then need to measure your tire size or just go online and determine it by the online sites. There are tons of speedo calculators out there. Try and get a gear combination that allows for some additional tweeking the driven gear high and low if you want to fine tune it. You should be able to get all the parts before taking the tail housing off. I got my parts at a trans shop (old set). On the drive gear there is a funny "J" clip that seems to get easily lost.
     

  5. Kerrynzl
    Joined: Jun 20, 2010
    Posts: 2,906

    Kerrynzl
    Member

    As above,
    But you need a bit more information about whats already in there.

    You need to raise your speedo ratio, but there is inconsistancies with your numbers.
    At 55 it's showing 45 which is 22% too slow[82%], but at 84 its showing 65 which is 29% too slow [77%].
    Ratios are consistant [ or linear ] so the percentages shouldn't change

    You need to get the number more accurate [ try a GPS ]

    Here's a link to how to calculate and get the correct ratio if you already know what the existing ratio is. The calulation method is the same but you need to reverse the numbers to speed it up

    http://www.jalopyjournal.com/forum/showthread.php?p=7720853#post7720853
     
    Last edited: May 1, 2012
  6. Jimv
    Joined: Dec 5, 2001
    Posts: 2,924

    Jimv
    Member

    Mine does the same thing!! I can't seem to find a gear with less then 15 teeth, i think i need a 13.
    I just keep a tomtom in the car & use that for a speedo to calibrate speed at times!!lol
    Jimv
    PS on another note, i've been driving my car for 14 yrs and it has 61,000 miles on it, but also reads 1 mile when i go .8 of a mile so does that mean i have 72,000 miles on the car?
    ( i think thats how its reading, my speed is higher then my speedo says)
     
  7. mike hohnstein
    Joined: Dec 4, 2011
    Posts: 262

    mike hohnstein
    BANNED
    from wisconsin

    You're off 20%. T-350s are the easiest trans to dial in the speedo, lots of gear ratios, but some times you need one of those expensive custom adapters done up by a speedo shop.
     
  8. Joe H
    Joined: Feb 10, 2008
    Posts: 1,525

    Joe H
    Member

    If the magnet drive in the speedometer head is weak, the ratio can be off the faster you go. A better way to check is to use the odometer, its gear driven and the ratio will never change. Drive a steady speed on a marked highway, 10 miles or more. that will give you enough information to change out the gears.

    Joe
     
  9. eldojoe
    Joined: Oct 27, 2010
    Posts: 3

    eldojoe
    Member
    from Wisconsin

    Thanks to all of you HAMBERS for the FAST responses! The car is up on the hoist for a lot of work to do to get it ready for the season, so the fast help really is great! Special thanks to Lubocrod for getting the basic "More Teeth or Less Teeth" concept straight in my head. And Huge thanks also to Dane for the TCI calculator link that had not just math but actual part numbers and colors! Now I can buy what I need and get the job done pronto.
    Aquarosco: I appreciate the tip to look inside with a flaslite. I'll be able to keep the Drive gear in the tranny as is.
    Kerrynzi/ Mike Hohnstein? Joe H: Thanks for the other tips including the GPS checking. Good useful stuff! I'll use it all.
    Once again, thanks for the timely help! Joe
     
  10. squirrel
    Joined: Sep 23, 2004
    Posts: 55,942

    squirrel
    Member

    I get the gears set up so the odometer reads correct, then if needed I play with the speedo calibration, there is a spring anchor adjustment you can make as well as adjusting where the needle points.
     

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