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Technical Speedometer calibration

Discussion in 'The Hokey Ass Message Board' started by junkyardjeff, Sep 16, 2018.

  1. junkyardjeff
    Joined: Jul 23, 2005
    Posts: 8,588

    junkyardjeff
    Member

    Took the 460 and C-6 out of the F100 and replaced it with a 351-W and C-6 and the speedo is 5 too slow,I thought I was told each tooth of the gear is good for 3 MPH so I was thinking 2 less teeth and if I can get 1 fast or slow it will be close enough to me so is that right. The last time I corrected it when it was slow I went from 2.75 to 3.00 gears and it was 1 fast but the 3.00 gears are staying and would like to change the speedo gear only this time..
     
  2. squirrel
    Joined: Sep 23, 2004
    Posts: 55,935

    squirrel
    Member

    how is the odometer?

    The amount adding/subtracting a tooth will affect it depends on how many teeth it has...it's a percentage, you know
     
  3. junkyardjeff
    Joined: Jul 23, 2005
    Posts: 8,588

    junkyardjeff
    Member

    If the speedo is slow I presume the odometer is too.
     
  4. squirrel
    Joined: Sep 23, 2004
    Posts: 55,935

    squirrel
    Member

    well....usually the speedo is not quite calibrated right, relative to the odometer. I like to get the odometer right by adjusting the gears in the trans, then fiddle with the speedo spring and needle position to get it to read correctly relative to the odometer.

    but I play with this stuff way too much
     
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  5. deucemac
    Joined: Aug 31, 2008
    Posts: 1,483

    deucemac
    Member

    I ran a dyno at a couple of Ford dealers. Generally, I found that 1 tooth chang in the driven gear was worth 2 1/2 mph. I used to love when calibrated their speedo after they got a ticket. I'll show this to the judge and get out of the ticket was their usual statement. To which I would reply, I don't think so, usually the judge would say something like "congratulations, now you shouldn't get another speeding ticket, pay the clerk".
     
  6. junkyardjeff
    Joined: Jul 23, 2005
    Posts: 8,588

    junkyardjeff
    Member

    I will pull the cable tomorrow and count the teeth and see if I have one with two less teeth and if I do not see if a dealer can get me one.
     
  7. s55mercury66
    Joined: Jul 6, 2009
    Posts: 4,335

    s55mercury66
    Member
    from SW Wyoming

    My brother is into King Midgets, and during his efforts to calibrate his speedo to his modified drivetrain, came upon some information stating that most speedometers are calibrated to read 60 mph at 1000 rpm, that being speedometer drive cable rpm.
    In the late '60's, the new owner of the King Midget Corp. decided to certify the cars for a 50 mph cruising (lol) speed, so they could be run legally on the Pensylvania Turnpike, among other roads with minimum speeds. His first trip into PA resulted in a ticket for failure to maintain that speed, even though his newly "calibrated" speedometer indicated he was.
     
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  8. squirrel
    Joined: Sep 23, 2004
    Posts: 55,935

    squirrel
    Member

    yes indeed, 1000 revs of the speedo cable per mile is a rather standard number. I just assume that everyone knows that...thanks for mentioning it.
     
    BJR likes this.
  9. BJR
    Joined: Mar 11, 2005
    Posts: 9,814

    BJR
    Member

    So... 1000 rpm would equal 60 MPH on the speedo head? Am I thinking right?
     
  10. squirrel
    Joined: Sep 23, 2004
    Posts: 55,935

    squirrel
    Member

    yes, it would.
     
    BJR likes this.
  11. s55mercury66
    Joined: Jul 6, 2009
    Posts: 4,335

    s55mercury66
    Member
    from SW Wyoming

    As a matter of fact, Jim, I think you have mentioned that at some point in another thread.
     
  12. junkyardjeff
    Joined: Jul 23, 2005
    Posts: 8,588

    junkyardjeff
    Member

    It had a 19 tooth gear and I had a 16 tooth gear and according to digital the your speed is sign its right on.
     

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