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1G 91 trans in a 90 speedo off

Posted by TK9er, Mar 27, 2020

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  1. TK9er

    TK9er Proven Member

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    I was wondering if there is a way to get my speedo to read right. My car is a 90 and it has a 91 trans in it. It is about 7 - 10 off. I am running stock 2 g rims. Since the dashes are different, I just assumed it was the trans that is making it read different. Thanks!
     

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  2. 19Eclipse90

    19Eclipse90 DSM Wiseman

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    I'm pretty certain the transmission is not the issue. The transmissions use the same speedo gear and cable. And I've done this without any issue, myself. The gauges are marked differently but they aren't scaled any differently.
     
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  3. DSMPT

    DSMPT Proven Member

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    Need a little bit more details.
    7-10% off or 7-10mph/kph off? How did you check it's off? And where are you reading the VS? At the gauge or logging the signal? Are you running a standalone or stock ecu box?
    If you mean that it's off at the speedometer, then the issue would probably be something between the speedometer and the transmission or the tire outer diameter (Not rim diameter). But as @19Eclipse90 mentioned that the speedo gear ratio and the cable is the same between 90 and 91+, first I would doubt the speedometer malfunction or the difference from the tire OD.
    If you are checking the VS at ECU (and if reading correctly in the speedometer), then the issue would probably be between the speedometer and the ECU. If you are using some standalone ECU, maybe the VSS calibration is off.
     

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  4. TK9er

    TK9er Proven Member

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    I am talking about the mph on the dash. I checked it against a gps app. My ecu is a stock with ecmlink. I haven't check the speed at the ecu. I'll do that.

    So, do the cables go bad?
     

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  5. DSMPT

    DSMPT Proven Member

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    My guess would be the GPS speedo is the one not accurate. If the cable or the speedo gear is bad, you would probably see some weird movement in speedometer. But better check it physically. It won't cost anyways.
     

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  6. TK's9d2TSi

    TK's9d2TSi Supporting Member

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    The cable runs to a gear and it reads the speed off the front diff. If it’s smooth, it should be fine. What size tires are you running?
     

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  7. 19Eclipse90

    19Eclipse90 DSM Wiseman

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    To expand on @tk106 's question, there are calculations to help you estimate what speeds you should be traveling at in any gear at a certain RPM, based on your tire size.

    I put this together a while ago as a reference and was using gear ratios which I had found to be the same for 1G and 2G AWD cars with a manual transmission, as well as the GVR4. This chart would be fairly accurate for the 1G AWD with a manual transmission when 25in diameter tires are used. As a reference, a stock 1G wheel used a 205/55/16 tire which should be about 24.9 inches in diameter.

    DSM MT AWD Speed-Gear-RPM EST Relationships.jpg
     
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  8. TK9er

    TK9er Proven Member

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    I just checked my tires. They are 225/50r16... I'm guessing that is where my problem lies... Would you all agree? 19Eclipse90 mentioned that a stock tire size is 205/55r16

    Never minds... I just checked. Those are basically the same tire
     
    Last edited by a moderator: Mar 31, 2020

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  9. TK's9d2TSi

    TK's9d2TSi Supporting Member

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    Yeah that won’t make a noticeable difference.
     

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  10. DSMPT

    DSMPT Proven Member

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    Those tires don't make a big difference.
    I don't know at what speed you are saying 7-10mph off. The tire diameter is always the first thing to suspect but if the tire OD is the issue, you always see the same % is different from the actual speed.
    The point here is you are making the GPS speedo as a criterion. That may not be accurate.
    You can calculate the VS with the tire OD and gear ratios. So calculate (each gear, each 100RPM or so) and go drive to log the VS with ECMLink, and then check if the VS in the log is following the calculated numbers. If they almost match then the GPS is not accurate.
     

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  11. TK9er

    TK9er Proven Member

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    I'll try and do this tomorrow. But I do have to ask, does VS stand for vehicle speed???
     

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  12. DSMPT

    DSMPT Proven Member

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    Yes, vehicle speed.
     

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  13. TK9er

    TK9er Proven Member

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    Well, I finally got out and checked my speed on ecmlink and it is reading the same as the GPS. It's about 7 mph off. Any ideas?
     

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  14. DSMPT

    DSMPT Proven Member

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    So the ECU is correctly reading the VS, and the ECU VS and the GPS VS match. I now believe that your speedometer needle position is the one off. If so, you need to adjust its position.
     

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  15. TK9er

    TK9er Proven Member

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    You can adjust the needle on the speedo? That's a game changer!!!
     

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  16. DSMPT

    DSMPT Proven Member

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    Have you or the previous owner ever removed the speedometer needle to install an aftermarket glow gauge or back ground? If it has done, the zero position may be set incorrectly. It's just remove the needle and set it in the right position. Can you upload a pic of your speedometer?
     

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  17. ist dwa

    ist dwa Proven Member

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    If you take apart the gauge cluster glass you can pop off the needle and reposition it to where you feel is best. The odometer reset button just pulls out, the rest is self explanatory. Post a pic of the gauge as it is now?
     

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  18. TK9er

    TK9er Proven Member

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    Attached Files:

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  19. DSMPT

    DSMPT Proven Member

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    I imagine always yours is showing approx. 7mph slower than the actual VS. Please check the pic below, that is my spare 90 cluster. The needle is set at "5" that is the zero position and which is actually "0" in 91-94 cluster. It seems yours is set at "0" in 90 cluster, that is what I meant earlier. "5" is the zero position in 90 cluster.

    DSC_1924.JPG
     
    Last edited: Apr 10, 2020

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  20. TK9er

    TK9er Proven Member

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  21. DSMPT

    DSMPT Proven Member

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    I guess someone has removed the needle before you or something is wrong inside mechanically.
    Remove the cluster and the clear cover. Carefully remove the needle with a fork or something like that, then put it back to the zero position (at 5 mph). Then put some tiny screw driver or so into the hole behind the cluster where the cable goes. If you turn the screw diver counter clockwise, the needle goes up. If you turn it clockwise the needle shouldn't move if you have correctly put the needle at the zero position. If the needle moves to minus side, you are not putting the needle in the correct position or the speedometer is broken.
     

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