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  1. #1

    Default Speedometer gear help, 4-speed 2.3L (hummer transmission)

    Looking for any direction with speedometer gear replacement as my speedometer is off by ~5mph below 40mph and 15mph about 55mph.

    Every where I've have found has talked about the five speed speedometer gears but none on the 4spd.

    Thank you in advanced,

    Tyler

  2. #2

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    I have some conversion charts for gears for the 4 speed on the 5.0 transmissions and I really believe they are different from the 2.3. I think even experts would need more information from you though. What year is the car? What is the tire diameter? What is the differential ratio? All of those things affect the speedo for sure. I suspect we also really need to know how many teeth are on the gear that is on the end of the transmission speedo cable as well. I know you said what engine and transmission it has, but obviously on a car of this age, it could have been changed out to who knows what.
    Good luck.
    Liberty once lost is lost forever.

    John Adams
    July 7, 1775

  3. #3

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    25.16 in diameter tire. 215/60r15. 1986, 60k on the body. Door tag says it's a hummer transmission "7" under the TR section. I would have to take the cable out to check tooth count, just wondered if it was something someone could answer with the information given.

    Thanks,

    Tyler

  4. #4

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    Off which way? Speedometer reading too fast, +1 tooth driven gear. Speedometer reading too slow, -1 tooth driven gear.

    Talk all over about this transmission or that one doesn't mean anything, because there's no way of doing the math without knowing how many drive gear teeth there are on the transmission's output shaft, which requires a lot more work than pulling the cable and gear out for a look.


    The math:

    1. Drive tires revolutions per mile = 5280 / ( ( pi x tire Ø ) / 12 )

    2. Speedometer driven (cable) gear =
    ( # of drive gear teeth on transmission output shaft x rear axle gear ratio x rear tire revolutions per mile ) / 1000


    Example: P215/60R15 = Ø25.16" = 801.68 revolutions per mile

    ( 7 drive teeth x 3.08 rear axle gearing x 801.68 revolutions per mile ) / 1000 = 19.36
    This situation would use a 19-tooth speedometer cable gear.
    Last edited by Walking-Tall; 01-14-2017 at 01:59 AM.
    Mike
    1986 Mustang convertible ---> BUILD THREAD
    Past Fox-chassis "four eyes":
    1983 Mercury Cougar LS
    1986 Ford Thunderbird ELAN
    1980 Capri RS Turbo

    Work in progress website ---> http://carb-rebuilds-plus.boards.net/

  5. #5

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    OK. I'll have to search a thread for details on how to get the driven gear count on my trans.

    The speedometer reads fast.

    Thank you for the insight.

  6. #6

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    Probably some time next week, can I jack up the rear of the car and turn the tires slowly with some help from a buddy and count the driven gear teeth?

  7. #7

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    You're welcome, Tyler.

    No, you can't find the # of drive teeth on the transmission's output shaft like that. Finding out the # of drive teeth on the transmission output shaft requires removing the drive shaft and then the transmission tail stock and then putting eyes on it and counting the teeth. They are normally 6, 7, or 8 teeth.

    Your speedometer isn't radically off. I've corrected ones much further off, like when I put 3.73 axle gears in place of the 2.73's in my Thunderbird, where I had to replace the factory 16-tooth driven gear with a 21-tooth driven gear, because the speedometer was reading WAY fast, lol.

    I would jack up the driver's side of the car, put axle stands for safety at the front and rear, find and follow the speedometer cable to where it enters the transmission, undo the (5/16 or 3/8?) bolt that holds it in, pull it straight out and see which plastic driven cable gear you have, then call or make a trip to your local Ford dealer and I'd imagine getting the next greater tooth count driven gear (ie. if yours has 18 teeth, get and install a 19 tooth gear) for your vehicle should get the speedometer reading right or closer. If you are certain about your speedometer being off, too fast, there isn't anything else that can be done besides going to the next higher tooth gear anyway to correct it... good luck with it.
    Mike
    1986 Mustang convertible ---> BUILD THREAD
    Past Fox-chassis "four eyes":
    1983 Mercury Cougar LS
    1986 Ford Thunderbird ELAN
    1980 Capri RS Turbo

    Work in progress website ---> http://carb-rebuilds-plus.boards.net/

  8. #8

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    This website has an application chart about transmissions and speedo gears.
    http://www.americanmuscle.com/speedo...th-manual.html

    (Assuming this is an early Fox speedometer.)
    The speedometer itself can be adjusted/calibrated. The tricky part is how you would test it. How do you know how far off it is ?

    This is a speedo with the cover plate removed.
    Name:  speedo flat spring.jpg
Views: 22
Size:  72.7 KB
    That flat spiral spring controls the movement of the needle.
    Adjusting the tension of that spring will calibrate where the needle points when the speedometer mechanism spins.
    Rotating the little plate the spring mounts to will increase or decrease the spring tension.
    Access to the adjuster is from the back through that cutout or from the side under the cover plate.

    The factory (I imagine) would spin the unit on a bench at a known precise rpm and then adjust the spring until the needle points to the correct place.
    If you try this it will be trial and error. Make a little adjustment and see what happens.
    (which is a real PITA to R&R the cluster over and over )

    If your's is reading too fast there needs to be more tension on the spring. (adjuster would go counter clockwise)
    ****Try adjusting 1/16" at a time. No more than 1/8" *****
    Last edited by jessesandy; 01-15-2017 at 08:46 PM.

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