Replacing speedometer driven gear

Discussion in 'The "Juice Box"' started by GranSportSedan, Jan 28, 2019.

  1. GranSportSedan

    GranSportSedan Puppet dictator, wife has all power

    I have a transmission that came from the factory specified for a 3.23 gear ratio car. The transmission is going to be in front of a 3.42 ratio rear. Is it fairly simple to just change the gear that drives the speedometer cable to get the correct speedometer reading ?

    BRUCE ROE Well-Known Member

    Yes, can be done in the car. I would check how accurate the car is now,
    correct any error as well as take care of gear change in one step. It
    might require both drive and driven gear changes, possible in the car
    on most earlier transmissions. A extreme gear ratio sometime needs
    an external gear box to get in range. Bruce Roe
  3. LARRY70GS

    LARRY70GS a.k.a. "THE WIZARD"

    Need 3 numbers, rear tire size (height), rear gear ratio, and number of teeth on the speedometer DRIVE gear.

    20168/ tire height = Tire Revs per mile (TRM)

    TRM X Ratio X Tooth count of DRIVE gear/ 1001 = DRIVEN gear tooth count.

    For a 1970 GS455 Stage 1 stock

    P245/60R-15 tires at 26.6" tall

    3.42 Ratio

    16 tooth DRIVE gear

    20168/26.6 = 758.195

    758.195 X 3.42 X 16/1001 = 41.44 or 41 tooth DRIVEN gear required.
    Last edited: Jan 29, 2019
  4. GranSportSedan

    GranSportSedan Puppet dictator, wife has all power

    Transmission is currently out of the car so now would be the time to make the adjustments if possible.
  5. DasRottweiler

    DasRottweiler -BuickAddict-

  6. LARRY70GS

    LARRY70GS a.k.a. "THE WIZARD"

    Why not? It's possible.
    GranSportSedan likes this.
  7. GranSportSedan

    GranSportSedan Puppet dictator, wife has all power


    BRUCE ROE Well-Known Member

    The formula 20168/ tire height = Tire Revs per mile (TRM) would be true
    if your tire made contact as a perfectly round disc. However tires are FLAT
    where they contact the road, flexing (varying length) coming off contact.
    The real ROLLING circumference is determine by the belt inside, so the
    above formula will always err.

    There is no need for trial and error with a working setup. Just make a
    precise measurement of current error (GPS, calibrated mile markers, etc)
    and then calculate the correction needed on your current gears, you will
    be precisely on the first time. Bruce Roe
  9. LARRY70GS

    LARRY70GS a.k.a. "THE WIZARD"

    I have yet to have someone explain to me how the circumference of the tire changes. The circumference of a circle is Pi X Diameter. I don't understand how that Changes just because it flattens out at the contact patch. Does it stretch, does it shrink? The tire revolutions per mile are based on the circumference. If there is an error, is it really worth talking about? I have always used the formula for calculating the DRIVEN gear necessary. Maybe I am wrong or I have a mental block about this:D. I just don't get it.

    My Speedometer is driven off the Gear Vendors. I followed the chart in the instruction manual and my speedometer is dead on according to the GPS. It was just as accurate when it was driven off the transmission and I used the formula.

    From the GV manual if anyone is curious.

    Last edited: Jan 30, 2019
  10. wkillgs

    wkillgs Gold Level Contributor

    Well, they're rubber! Air pressure, speed, tread wear.... all minor unless you're talking top fuel!
    I had figured 1,000 speedo cable revolutions per mile, but I think the auto makers use a slightly different figure, maybe for slippage?

    Bob, note that the gear holder (sleeve) is meant for a certain range of driven gear sizes. Sometimes you have to change the sleeve if the gear is too big/small.
    GranSportSedan likes this.
  11. LARRY70GS

    LARRY70GS a.k.a. "THE WIZARD"

    I get that, centrifugal force might have the tire grow at speed which would increase the rolling circumference, but the fact that the tire flattens at the contact patch has nothing to do with that? Slippage, that has to be pretty small, not sure that's even worth considering.

    The formula isn't perfect, but it is easiest IMO. Even the factory used ratio adapters for some combinations. That is definitely the only option for some not willing to change a DRIVE gear that makes a DRIVEN gear not available.

    BRUCE ROE Well-Known Member

    I will try to explain in detail when I get back from mailing some owners
    car parts. In the mean time, some research might turn up "rolling
    diameter" that some tire makers release. Bruce Roe
  13. DasRottweiler

    DasRottweiler -BuickAddict-

    For our purposes, the calculators available at TCI website are more than sufficient . Jim/Rott
    GranSportSedan likes this.
  14. DasRottweiler

    DasRottweiler -BuickAddict-

    Just found this, may be helpful.....Jim
  15. GranSportSedan

    GranSportSedan Puppet dictator, wife has all power

    the outside circumference of the tire isnt going to change enough to make the speedo reading change. so a 26 inch tire has 81.61 inches of tread and that stays the same even if the tire went oval or heaven forbid ...square.

    BRUCE ROE Well-Known Member

    1001 speedo cable turns per mile is to make the odometer correct,
    which has reduction gearing 13:1, 11:1, and 7:1. Multiplies to 1001:1
    As I recall the target for the Metric version is 616:1

    We know the factory was not super keen on accuracy, and I am not
    telling anyone how accurate is good enough. But I will tell how I made
    my car as accurate as possible.

    Regarding the flat side of a tire. Imagine a vehicle with tracks instead
    of wheels, it always has a flat section of track on the ground, the rest of
    the track shape does not matter. Imagine a pin was pulled and the track
    laid out flat on the ground. If we measure the exact distance between the
    holes the pin goes in at each end, we could agree that is the distance
    travelled when the track goes around exactly once.

    THEN consider this. The above said nothing about how high the pin is
    above the ground. In fact, if the track wore down so the pin was an inch
    closer to the ground, the measured length would be EXACTLY THE SAME.

    That is how tires work. The "PIN" is at the level of the fiber belt in the
    tire, not at ground level. The tread is not continuous, but flexes as it
    meets and leaves the ground. The outer surface of the tread is not at
    the effective dia, the belt is. I came to realize this messing with transmissions
    in the 70s. A car I modified came mostly by accident, to be about a decimal
    place more accurate than normal. I could (did) drive 50 miles by mile markers,
    and they would still be exactly in sync with my odometer. But surprisingly,
    when those new tires wore down, THE ACCURACY WAS UNCHANGED.
    The final evidence was when a different brand of tire (same size) made a
    measurable shift in my carefully checked readings, because its belt was not
    done exactly the same way.

    In time I measured some local roads to 1/100 mile, so I did not need to go
    on a toll road for each check. Using the usual formula will get you in the
    ball park, except any input error will blow it. How do you get the tire dia
    anyway? But if you measure error of what you have, the required
    correction is easily and exactly calculated. My PHOTOBUCKET album
    SPEEDOMETERS AND GAUGES shows some of this, including how to
    roll your car forward one tire rev and measure its effective circumference.
    Probably not as precise as at speed.

    click on an Album
    click on a picture to enlarge + description

    Bruce Roe
  17. wkillgs

    wkillgs Gold Level Contributor

    Thanks for the explanation Bruce. When I was a toddler, I always asked 'why?'.... I never out grew that!

    Nice solar array! Have I seen you post on the SolarPanelTalk forum? Thought I saw your name there.

    When changing gears to correct speedo error, I usually figure 1 tooth on the driven gear for 3 mph. Is that a decent ballpark for a TH300/400?

    BRUCE ROE Well-Known Member

    I can not give you direct teeth per mph, it would depend on the
    drive gear and also the axle ratio. Look in there and get the tooth
    count on both speedo gears to get the ratio. Measure the error on
    the road, figure the ratio you really need, and pick new gears to get
    the new ratio.

    Yes my solar supplies all my heat and electricity here west of Chicago.
    Bruce Roe
    . Ap18PV1.JPG
  19. HotRodRivi

    HotRodRivi Tomahawks sighted overseas

    what about cable stretch and flex? If you burn rubber a lot you will make your tire smaller too. Thats why you need STEEL TIRES!. Yes the new tire that stays hard always.

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