Speedo is way off !?!

Discussion in 'Fox 5.0 Mustang Tech' started by dirty slut, Jan 16, 2006.


  1. dirty slut

    dirty slut New Member

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    So supposedly I have 3.73 gears from one of the previous owner's, but w/ OEM size tires, 225/55/16, my speedo is 10mph off:notnice:
    I go to put in the 23 tooth white speedo gear to correct the issue and there is already one in there:notnice:

    So what gears are in there? Would 4.10s be 10mph off? When the speedo says 70, im actually going 60, this is of coarse give or take a few MPH's as I had my wife drive next to me

    Edit:
    92 LX
    T-5 tranny
    #1
  2. Paully's5.0

    Paully's5.0 Founding Member

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    you're in the same boat as I am. I "supposidly" have gears in the rear from a previous owner, and my speedo is off as well.

    The problem is that there is another gear inside the tranny, its 6,7 or 8 tooth. Unless you know how many teeth are on that gear as well, you really won't be able to figured out for sure what your rear gears are.

    You could try jacking up the rear, have a buddy turn a wheel while you count the number of times the drive shaft spins, per one turn of the wheel. Not very accurate but it might help to verify things.

    #2
  3. 5.0 Nostalgia

    5.0 Nostalgia Member

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    What year is your car and what transmission do you have? The gear inside the transmission was changed in 90 or 91.
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  4. willys1

    willys1 Banned

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    ahhhh MAN!!!! All I saw was the word "speedo" written by Dirty ****!
    Gotta get my mind out of the gutter!!:nonono:
    #4
  5. dirty slut

    dirty slut New Member

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    Its a 92 LX w/ the manual T-5 tranny.
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  6. sunil6784

    sunil6784 Member

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    Check to make sure they really are 3.73's
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  7. dirty slut

    dirty slut New Member

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    How do I do that?
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  8. 5.0 Nostalgia

    5.0 Nostalgia Member

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    They probably are 3.73's. In my 91 coupe, I needed the 23 tooth speedometer gear to get my speedometer correct with 3.55's. My 89 with 3.73's and the same gear is accurate. The only way to fix it is replace the speedometer gear in the transmission with the drive gear from an 87-89 t5. Honestly, it is more work than it's worth.
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  9. speedytang

    speedytang Founding Member

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    Remove your speedometer gear and look in the hole to see what color your internal gear is. It is probably white which means you have no gear choice from anything over 3.55 gears and a 23 tooth speedo gear. 87-89.5 transmissions have the internal yellow and you can use 3.73 without going to the weak 23 tooth speedo.
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  10. Michael Yount

    Michael Yount Advanced Member

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    What you'll need to do is remove the tailshaft of the tranny and change the DRIVE gear to a 7 or 6 tooth. That will allow you to install the correct driven gear to correct your speedo. I've done this on mine - twice in fact - it's a bit of work. Both times I already had the tranny out of the car. 7 tooth should work with a red 21 tooth driven gear; 6 tooth is better with an 18 tooth yellow driven gear.

    If you don't want to go that route, then you can get a speedo shop to make a 'black box' for you. It has an input for your cable, and an output for the tranny. It will allow you to swap out gears in the black box and make about any correction you want to.

    Here's a link with good info on what gears came with what tranny's and what years.

    http://www.geocities.com/mgnedell/notchback/tech/speedo.html
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  11. dirty slut

    dirty slut New Member

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    ^^that is the chart I used to figure out that I needed the 23 tooth white gear. It says it its not 100% accurate but I figured it would only be off by a few MPH's, not 10. So is there another tooth I should try to correct it. Would the 21 tooth red one slow it down by 10MPH?
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  12. Mike92GT

    Mike92GT Founding Member

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    I thought the stock black speedo gear was a 19 tooth? Is it actually 20? I'm also in the process of trying to get my speedo corrected.
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  13. 5.0 Nostalgia

    5.0 Nostalgia Member

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    Don't use the 21 tooth. That will make it off by more than 10 mph. You actually need more teeth on the driven gear to make it slow down. Just like with rear end gears. The less teeth, the faster it spins. However, a 23 tooth is the most you can buy. And they are more expensive. And they break. Your only option for accuracy is the drive gear in the trans or the calibration box that Michael suggested.
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  14. Michael Yount

    Michael Yount Advanced Member

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    **** - If you hold the number of teeth on the DRIVE gear constant, when you increase the number of teeth on the DRIVEN gear, you speed the speedo up. When you decrease the number of teeth on the DRIVEN gear, you slow the speedo down. So, dropping from 23 to 21 is only gonna make it worse. Remember - it probably had a 17 or 18 tooth on their to start with.

    So, since 23 teeth on the drive gear won't do it, your only alternative is to DECREASE the number of teeth on the DRIVE gear. That will also speed the speedo up. Your ideal is a 6 tooth drive gear and an 18 tooth driven gear. Should be pretty accurate. But pulling the tail shaft housing off may require dropping the tranny....
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  15. dirty slut

    dirty slut New Member

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    Hmmm, ok, I dont wanna go that route, dont feel like droppin the tranny. So what's the general consensus here. Do you think I have 4.10 gears or what. The previous owner said they were 3.73s but I dont know. I really dont know what gears are in there
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  16. 5.0 Nostalgia

    5.0 Nostalgia Member

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    You have pretty much proven that you have 3.73's by what you have posted here. On a 90 and up, if you have 3.73's with a 23 tooth gear, your speedometer will be about 10 mph off if you have the 8 tooth drive gear. My old 91 GT had 3.73's and was about 10 mph off with the 23 tooth gear. On my 91 coupe, I needed the 23 tooth gear with 3.55's to make the speedo accurate.
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  17. Michael Yount

    Michael Yount Advanced Member

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    You never want to try and determine rear end ratio the way you are -- too much room for tach error, speedo error, tire size changes, etc.

    It's simple to know what gear is in the car -- jack up the rear end. Have a friend rotate the tire exactly one rotation while you count the number of times (carefully) that the driveshaft rotates. If it rotates just a bit over 4 times for one tire rotation you've got 4.10's. If it roatates about 3 3/4 times - you've got 3.73's.
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  18. jerryD

    jerryD Founding Member

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    tell us what your taching at on the hiway with the speed of another car's speed that is known. that may help out some too.
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  19. Michael Yount

    Michael Yount Advanced Member

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    Jerry - the stock tachs in the Stangs are notorious for being inaccurate - so even that method isn't foolproof. Count rotations - it's easy; it's accurate.
    #19
  20. jerryD

    jerryD Founding Member

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    didn't know he still had stock tach.
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