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Thread: Speedo off?

  1. #1
    Join Date
    Mar 2010
    Location
    Grafton, OHIO
    Posts
    651

    Speedo off?

    Just changed out the 3.27's for MK VIII housing with SC freshened T-loc and 3.73's with 245/45-18's on my '90 XR7. Read up on the speedo gears and purchased 23 tooth from Late model Restoration for mustangs. There was a 23 tooth driven gear for 7 tooth or 8 tooth drive gear (2 different gears ava.). According to info from TCCoA we have an 8 tooth drive gear, so I purchased the recommended gear according to their calculator.

    The speedo appeared to be at least close for the first 1/2 a dozen short tests. After about 30 miles it suddenly jumped to being low by around 20-25 MPH at highway speed. It shows me at 45 mph @ 2200 rpm, and I am running right with traffic, so I am sure I am at least 60-65. In the recent past the speedo DID act very strange a couple times, and I THINK it may point to my problem. sitting at idle in park, the speedo would suddenly jump up and read some speed, any where from 20 to 65 at various times.

    So can I get verification we DO have an 8 tooth drive gear? I understand the wrong gear will blow the teeth off the driven gear in a hurry. The short period of correct operation makes me wonder about this. The calculator at LMR also seemed to assume a 7 tooth gear for MUSTANG AOD's. The TCCoA article stated we have an 8 tooth, and I do trust that info, BUT...

    My next thought is that in disturbing the old electronic speedo sending unit it may have suddenly decided to throw in the towel and is sending a low output. In research here I have also seen others with an actual speedo head unit problem. The odometer actually does still work on this unit, if that means anything. B4 I start just changing out parts is there a quick way to pinpoint the head or sending unit as the problem?

    Thanks for any help or input

    Adam
    Last edited by aroot1; 05-03-2018 at 04:16 PM.

  2. #2
    Join Date
    Apr 2002
    Location
    Norcross, GA
    Posts
    3,319
    I do believe that we have the 8 tooth speedo gear. My 89 had a jumpy speedo so I changed out the entire sending unit and the problem cleared up so you probably do have a bad sending unit. I also have discovered that the 23 tooth gears don't hold up. The teeth are just too thin. Had one last only 50 mi before it stripped. The 21 tooth is off a little off, 5-7 mph, but it holds up better.
    90 5spd, best ever E.T. 12.250
    best ever 1/4mi speed 112.89 mph
    Current dyno tune 383.54 rwhp 418.88 rwt

  3. #3
    Join Date
    Jan 2010
    Location
    Bossier City, LA
    Posts
    2,232
    Quote Originally Posted by Mike Puckett View Post
    I do believe that we have the 8 tooth speedo gear. My 89 had a jumpy speedo so I changed out the entire sending unit and the problem cleared up so you probably do have a bad sending unit. I also have discovered that the 23 tooth gears don't hold up. The teeth are just too thin. Had one last only 50 mi before it stripped. The 21 tooth is off a little off, 5-7 mph, but it holds up better.
    There's two other ways to get around it.

    1) Dallas Mustang also makes the Speedcal, and other companies make the same basic thing ... it takes pulses from the VSS with the current gear and scales the frequency as per your settings to produce the appropriate frequency pulses out. https://www.dallasmustang.com/news/speedcal-faq/

    2) At least one company makes a MECHANICAL "SpeedCal" ... you give them the ratios, they build a gear train that fits, you install it. They also have canned sets. Here's one from my home town neighbors, I'm quite positive there's others out there: https://transmissioncenter.net/shop/...ratio-adapter/ (If I understand, you need to slow it down; they make one that also speeds the speedometer UP) (Disclaimer: These are my home town folks; again, as I stated, I'm SURE other folks make'm also.)

    With either of those, you can pick an even stronger gear (say an 18 or so) and correct the signal before it gets to the electronics. (I kind of like the mechanical; YMMV on that, though.)

    RwP

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