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Thread: speedometer off

  1. #1

    Default speedometer off

    I took my 86 coupe to get inspected and it is officially mine now.
    But I was driving on the freeway and noticed the speedo was off about 6 mph tach was at 2000 at 60 measured by another car
    I am pretty sure the 86 5 speed had 3.08 gears I haven't spun the tire to see which gears it has but does anyone know which gears it sounds like, I am thinking 3.55's

    Thanks

    Todd

  2. #2
    FEP Super Member cb84capri's Avatar
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    What did the other car measure when your car said it was going 60mph? What tires are on the rear of it?

    Cale

  3. #3

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    Mustang read 55 and actual speed was 61
    which is backwards I think

  4. #4
    Moderator wraithracing's Avatar
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    We have to make a few assumptions since you didn't provide the information.

    Assuming it's a V8 with stock T5 then the 5th gear is a 0.68 and assuming a 25" tall tire puts you at @ 61.62 MPH with a 3.55 gear

    Assuming the same above with a 25.7" tall rear tire that puts you at @ 63.35 MPH with a 3.55 gear
    ​Trey

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  5. #5
    FEP Super Member erratic50's Avatar
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    I would calculate off of 4th because 5th could be 0.68 for T5 or 0.63 for T5Z. I would specify the tire size.

  6. #6
    FEP Super Member erratic50's Avatar
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    I forgot to say.... 55 vs 61 is basically a tire size error. When I run 235/60/15 my speedometer would show 85 when I was running 92. Decreasing speed decreases the delta proportionally.

  7. #7
    FEP Senior Member dtmilsap's Avatar
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    LMR has a calculator that figures out which speedo gear you need, based on tire size and rear end gearing.

    Sent from my SAMSUNG-SM-G900A using Tapatalk

  8. #8
    FEP Super Member erratic50's Avatar
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    It doesn’t always work. You have to know tooth count on the gear inside the transmission. A lot of times it’s 7, but not always.

  9. #9

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    That sounds like 3.55s, my 86 is dead on 2000rpms at 60mph and it's had those gears in it since 312 miles were on it, one of the first mods I did.

  10. #10
    FEP Super Member erratic50's Avatar
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    Could be a needle calibration issue. How far does the odometer say you’ve went after 20 miles by GPS?

  11. #11

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    Thanks, I lifted it up last night and spun the tire and counted a little more than 3.5 driveshaft revolutions so I am guessing 3.55's which is a nice bonus.
    I am assuming it has the original transmission but until it gets torn down I really don't know much.

  12. #12
    FEP Super Member cb84capri's Avatar
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    I don't trust the stock tachometers. You never said what size tires are on the back of the car. You can pull the speedometer cable out of the trans and count the teeth on the gear, try to see in what color gear is in the trans. It should be a yellow 7 tooth gear in the transmission.

    Cale

  13. #13
    FEP Senior Member cb650's Avatar
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    Quote Originally Posted by erratic50 View Post
    Could be a needle calibration issue. How far does the odometer say you’ve went after 20 miles by GPS?
    I think mine needs that. The odometer is dead on but the speedo is 10% fast.

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