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Thread: Which speedo gear?

  1. #1

    Default Which speedo gear?

    So I have 4.10s (installed by po) in my 87 mgm. I used an online calculator and found that I needed a 21 tooth gear. I bought one and installed it today. Well the speedometer is still off the same amount. I pulled out a purple gear to install a blue one so I thought they were different but I just went to count the teeth on the gear I removed and it also has 21 teeth. So now Im a little lost, which speedo gear do I need? Id like to swap to 3.55s one day so if I could get a correct gear for those Id be happy.


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  2. #2
    Road Warrior Kodachrome Wolf's Avatar
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    Assuming a stock tire size of 215/70R15, you’d need a 24 or 25 tooth gear, but I have no idea if one like that exists. Might want to poke around LMR.com in their Mustang section. They might have what you’re looking for, or something close. There’s a chance you won’t totally eliminate your speedometer error though.

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  3. #3
    Beater gonna beat sly's Avatar
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    You'd have to swap in a 7 or a 6 tooth output shaft and then get the proper speedo gear for that to use 4.10s. The stock 8 tooth output shaft will spin the gears too fast. The other option is to get a gear adapter from a mustang LX that came with the automatic AOD and 4.10 gears or an aftermarket gear adapter.

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  4. #4
    Approaching 2 decades of DDing Box Panthers VicCrownVic's Avatar
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    There is a 23 tooth gear available: https://www.cjponyparts.com/speedome...hoC-EEQAvD_BwE

    I seem to recall hearing that these higher tooth count gears wear out after a while. One of the reviews seems to imply that this one might be better quality and not so prone to this issue.

    Another review says that the 23T gear is what you want for 4.10, BUT that is probably for a Mustang. Different size tires and transmission gearing in your MGM could mean the 23T gear still will not be accurate in your application (might be closer to correct though).

    Speaking of tires, different size tires could address the speedo inaccuracy, but I'm afraid smaller tires might be the direction needed to make the correction (haven't put much thought into it, so not sure), and that would be not a good solution I think.
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  5. #5
    Beater gonna beat sly's Avatar
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    yeah... 23 is for 3.73 on panthers. 21 for 3.55.

    Only two things are infinite, the universe and human stupidity, and I'm not sure about the former. -- Albert Einstein
    rides: 88 MGM (SOLD), 93 Vic, 2000 Crown Vic, 2003 Expedition
    Quote Originally Posted by gadget73
    ... and it should all work like magic and unicorns and stuff.
    Quote Originally Posted by dmccaig
    Overhead, some poor bastards are flying in airplanes.

  6. #6
    Beater gonna beat sly's Avatar
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    Here's an aftermarket item that requires an aftermarket speedo cable which will prevent you from using the stock VSS at the trans and require an in-line VSS.

    https://transmissioncenter.net/shop/...ratio-adapter/

    Or the 90 degree version: https://transmissioncenter.net/shop/...uous-rotation/

    they will both need the in-line VSS and LOKAR speedo cable (cut to fit or use the whole length and just tie it up)

    in-line VSS: https://transmissioncenter.net/shop/...edometer-head/

    The in-line VSS will require some rewiring of the car to adapt it.

    The easier to fit VSS will be the "behind the speedo" one from older Fords that is pretty much unobtanium. I think the guy I sold the Mark VI to may still have the spare I had and can get the part number from it to help with searching, but again with the rewiring stuff and you'd also need to find the short cable that goes from the speedo to the sensor.

    Only two things are infinite, the universe and human stupidity, and I'm not sure about the former. -- Albert Einstein
    rides: 88 MGM (SOLD), 93 Vic, 2000 Crown Vic, 2003 Expedition
    Quote Originally Posted by gadget73
    ... and it should all work like magic and unicorns and stuff.
    Quote Originally Posted by dmccaig
    Overhead, some poor bastards are flying in airplanes.

  7. #7

    Default

    Wow thanks for all the help guys. Im just going to leave it for now as I dont want to have to screw with it that much. Hopefully the 3.55s will be installed soon. In the meantime Ill just live with it.


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  8. #8
    Beater gonna beat sly's Avatar
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    yeah... If you're planning to go to 3.55 then leave it. Once you change the gears, you should be +/- 2mph no problem.

    Only two things are infinite, the universe and human stupidity, and I'm not sure about the former. -- Albert Einstein
    rides: 88 MGM (SOLD), 93 Vic, 2000 Crown Vic, 2003 Expedition
    Quote Originally Posted by gadget73
    ... and it should all work like magic and unicorns and stuff.
    Quote Originally Posted by dmccaig
    Overhead, some poor bastards are flying in airplanes.

  9. #9

    Default

    I spoke to a tech at Superior Transmissions a few months ago and he recommended 3.55s. I have a set thats just sitting and honestly I dont like the 4.10s. The shorter gear mixed with low shift points sucks.


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  10. #10
    I'm an air-conditioned gypsy gadget73's Avatar
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    4.10 makes no sense with the stock cam. The engine runs out of rev way too quick. The gov in the trans is appropriate for the stock crappy cam though. Extend the shift points with a stock motor and it just makes a lot of noise.

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  11. #11

    Default

    Exactly. The po installed a carb and 4.10s trying to make it a racecar lol. Needless to say, Im fixing a lot of things he did to it unfortuneately.


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