Speedometer/Odometer Trans Gear Question

Discussion in 'Got gears?' started by Chris Gaunt, Feb 8, 2020.

  1. Chris Gaunt

    Chris Gaunt Silver Level contributor

    I am running a TH 400 Trans. My rear tires are 81 inches in circumference and my rear end is a 3:73 Posi.
    Which Speedo /odometer gear do I need to have my speedometer read the correct speed?
    At the moment I am reading about 8 miles an hour faster than I should be.
    Any help would be appreciated.
  2. DasRottweiler

    DasRottweiler -BuickAddict-

    I have 3.42 gears with a 26" tall rear tire, TH400.
    The math worked out to a 15 tooth Drive gear ( on output shaft inside rear extension) with a 39 tooth Driven gear ( in speedo housing on side of trans).
    If I change my differential gears to 3.73s, then I will need a 42 tooth Driven /15 Drive gear combo .

    Try using the calculator here:

    15 tooth Drive gear:
    Check out this page from Summit Racing https://www.summitracing.com/parts/tci-880019a

    Probably a 2 to 3 beer job , depending on your drinking/ wrenchin' skills. Worth the effort to get speedo right.
    Last edited: Feb 8, 2020
  3. LARRY70GS

    LARRY70GS a.k.a. "THE WIZARD"

    81" in circumference equates to 25.8" in height. That's about an inch shorter than stock. Are you sure about that? Post the tire size on the sidewall. You'll also need to post the number of teeth on the speedometer DRIVE gear. That gear is on the transmission output shaft. You can see it through the sleeve hole. Rotate the drive shaft and count the teeth.
  4. Jim Jones

    Jim Jones Wretched Excess

    There are 63360 inches in a mile. A tire with an 81" circumference will revolve 782.22 times in that mile. At 60 MPH with a 3.73:1 final drive will result in 2915.55 RPM. Using the typical speedometer correction of 1001, the conversion ratio is 2.9126. So if your drive worm has the the number of teeth listed below you will need the corresponding driven gear:

    15 = 44 tooth driven gear.
    16 = 47 tooth driven gear (which is not available so an reducer would be required)
    17 = NA see above.
  5. Chris Gaunt

    Chris Gaunt Silver Level contributor

    I'm Running a 275/40ZR 17 98W tire. Thanks for your reply. It's a big help!

    Attached Files:

  6. Chris Gaunt

    Chris Gaunt Silver Level contributor

    Thank You Guy's I'm well on my way now with your help.

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