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Technical Speedometer reading fast

Discussion in 'The Hokey Ass Message Board' started by 1959 F100, Jun 4, 2016.

  1. 1959 F100
    Joined: Feb 21, 2014
    Posts: 121

    1959 F100
    Member

    The speedometer in My 1954 Buick Century is reading approx 5-10 mph fast. Speedometer gear in transmission is the original, never been touched. Speedometer working smoothly cable was lubed before I got it running a couple months back. I am running P225/70R 15 tires on the car. What should I be looking at to fix this?
     
  2. are those tires the same diameter as the originals?
     
    jeffd1988 likes this.
  3. 1959 F100
    Joined: Feb 21, 2014
    Posts: 121

    1959 F100
    Member

    No clue, My shop manual says it came with 7.60 15's
     
  4. Slow down approx 5mph...........
     
    C. John Stutzer likes this.

  5. Rumbullion
    Joined: Apr 3, 2016
    Posts: 15

    Rumbullion

    Are you still running 'tall' cross plies? Sounds like a size reduction with modern tyres, like smaller modern radials, even if they're still on 15 rims
     
  6. Could well have been off that far when new. And at what speed? 40, 70 MPH? And goodness knows what's going on after 60 years.

    Quick search says your current tires have a dia of 27.1 inches. The specced 7.60-15 have a diameter of 28.66

    Going to this page

    http://www.summitracing.com/parts/cok-59400/overview/

    and using the gear ratio calculator to solve for MPH using 3.23 axle ratio, 3000 RPM, and then plugging in the two different tire diameters, the result is a +/- difference of 5 MPH.

    Of course, you should use the actual numbers for your car, but you get the idea.

    Anyway, live with it, or go to an actual speedo shop and get it calibrated.
     
  7. BamaMav
    Joined: Jun 19, 2011
    Posts: 6,709

    BamaMav
    Member
    from Berry, AL

    2.25 is a pretty short tire. You probably need 2.35 or taller....
     
  8. 1959 F100
    Joined: Feb 21, 2014
    Posts: 121

    1959 F100
    Member

    Well, living with it got Me a $150 ticket for being 10 over without knowing, so sadly that's not an option. Speedometer was reading 20 mph, cop told Me his radar clocked Me at 30
     
  9. Davyj
    Joined: Jul 11, 2011
    Posts: 442

    Davyj
    Member

    S-10 had a gear reducer that went in between the trans and the cable. it will adjust the speedo
    http://www.ebay.ca/itm/Gear-Ratio-A...m542e81c5cd:m:mgNv9fZ5LB8zTxQP6Jsmnbw&vxp=mtr

    the link is to ebay , so you can see what it looks like. most junkyards have them and you can switch the gears around to go either up or down,
    or you can go to an old trans shop and see if they might have different speedo drive gears for your tailshaft.
     
  10. Mark T
    Joined: Feb 19, 2007
    Posts: 2,033

    Mark T
    Member

    Easy enough to recalibrate your speedo to the shorter tires, just change the gear pictured below. Less than $20 delivered on Ebay.

    [​IMG]
     
  11. Hnstray
    Joined: Aug 23, 2009
    Posts: 12,355

    Hnstray
    ALLIANCE MEMBER
    from Quincy, IL

    This is 180* opposite of your initial statement......"my speedometer reading fast". Your speedometer is reading SLOW if it shows 20mph and you are really doing 30 mph. And the error is 50% off at only 20 mph indicated!!

    Smaller diameter tires, as discussed in prior posts, would cause it to read 'fast'. Something is very inconsistent here.

    Ray
     
    Last edited: Jun 4, 2016
    Truck64 and C. John Stutzer like this.
  12. 3quarter32
    Joined: Dec 10, 2010
    Posts: 498

    3quarter32
    Member

    10mph off at 20mph would be a bit much for tire size difference don't ya think?
     
    C. John Stutzer and Hnstray like this.
  13. 29AVEE8
    Joined: Jun 28, 2008
    Posts: 1,384

    29AVEE8
    Member

    Until you can figure this out mechanically, there is a free iPhone app called "Speedometer free speed box" that will get you better off than you are.
     
    Hnstray likes this.
  14. 1959 F100
    Joined: Feb 21, 2014
    Posts: 121

    1959 F100
    Member

    Looks like the normal GM style speedo gear like in the TH 350 and TH 400, I can check with some area transmission shops, one might have some lying around.
     
  15. Yup. A 50% error at 20 mph is a lot more than a gear is going to fix as well.

    Of course the OP could have someone (or maybe 2 different) people with a recent factory stock car pace 20 mph and see who is lying - his speedo or the cop.
     
    Hnstray likes this.
  16. Davyj
    Joined: Jul 11, 2011
    Posts: 442

    Davyj
    Member

    borrow a gps with a speedo in it and verify the real speeds, put masking tape on the speedo and write it in...............then worry about it later :)
     
    Hnstray likes this.
  17. e1956v
    Joined: Sep 29, 2009
    Posts: 2,376

    e1956v
    Alliance Vendor

    You are not going to be able to correct the error with a driven gear change, it's to far off. By the police radar you're reading 33% slow.
    You need to run the car on a measured mile and see how many 1/10's you're turning in a mile, by the figures you gave you should be showing 7/10's in a mile.
    Run the car and give me the measured mile reading and I can build you a ratio adapter to correct the error.
    I explained the process in a previous post but couldn't figure out how to copy it to this thread, search I'm having speedo reduction issues and you can see how it's done and why it's best to use a measured mile as a base line.
    If your measured mile comes out correct then the speedometer needs to come out and be repaired.
     
    Last edited: Jun 5, 2016
  18. Skankin' Rat Fink
    Joined: Jun 18, 2006
    Posts: 1,496

    Skankin' Rat Fink
    Member
    from NYC

    This gear may not fit your transmission. I can say with certainty that Pontiac used a very different-looking gear in the 50s.
     
  19. Taller tires.
     
  20. Jalopy Joker
    Joined: Sep 3, 2006
    Posts: 31,175

    Jalopy Joker
    Member

    good offer. calculators to input gears, tires, etc are at sites like project33.com
     
  21. Assuming you're using a Windows desktop or laptop...

    Find your original post. Place your cursor at the beginning of the text you wish to copy. Click and hold the left mouse button while dragging the cursor over the desired text to highlight and select it. Release the left mouse button and click the right mouse button. In the dialog that opens, left click on "Copy" (or press Ctrl+C on the keyboard) to save your selection.

    Come back to this thread and open a "reply" window. Place the cursor where you want in the window, then right-click again. In the dialog that opens left click on "Paste" (or press Ctrl+V on the keyboard) to paste in your saved text. ;)
     
    e1956v likes this.
  22. How is your odometer working? If it is ok then you should consider having your speedometer rebuilt.

    Charlie Stephens
     

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