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    GM Speedometer Recalibration

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    Sweet Seventy9
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    Posts : 955
    Join date : 2013-11-03
    Location : Upstate NY

    GM Speedometer Recalibration

    Post  Sweet Seventy9 on Tue Sep 09, 2014 5:56 pm

    I found this on the internet a long time ago.


    GM Speedometer Recalibration

    Solve speedometer error by replacing the speedo gears in your trans
    New tires and wheels are a great addition to any car. The look and perform better than stock. Another popular modification to get your car to perform better is to install lower (numerically higher) rear gears. With wider tires, taller wheels and lower rear gears, the change is dramatic. However, so is the change in the speedometer calibration.

    The stock speedometer is calibrated at the factory to accurately measure mileage and speed using the stock tires and gears that came in your car. Engineers use the tire height and circumference along with the final axle ratio to get your speedometer to operate correctly. When you change the tire size and/or the axle ratio the speedometer needs to recalibrated to the changes.

    One way to correct the problem is to go to a professional and have the speedo reclibrated but that tends to be expensive. The cheapest way to fix the speedo error is to do it yourself. The speedometer is connected to the transmission via the speedo cable. At the end of the cable in the transmission are 2 gears; a driving gear (or trans gear) on the output shaft and a driven gear (or speedo gear) that the speedo cable attaches to. By changing one or both of these gears you change the speed of the cable. These gears are easily purchased from most GM dealerships with a good parts department. Changing the driven gear is easily accomplished by removing the speedo cable from the transmission, removing the lock bolt and sliding out the gear housing. Some gears are 3/4" in diameter and some are 1-1/2" in diameter. You'll have to measure which size you have and you can't mix the two. The driving gear can only be replaced by removing the transmission tail shaft.

    OK. Now you know who to fix the problem but how do you know what size gears to buy? Simple. Patrick Coxe has solved that problem by developing a formula and a table to determine the correct speedometer gears needed for a given combination of axle ratio and tire diameter. According to Coxe, the formula for determining the proper gears for any application consists of multiplying the rear axle by 20.2, and dividing that number by the tire diameter. (rear axle x 20.2 / tire diameter) Then, simply look up a ratio number on the on the following table (large and small gear versions) which is closest to the calculated number. By installing the gears, you should be close to the correct calibration.

    For example, I changed my Nova's axle ratio to 3.90:1, and installed 26-inch tall tires. Multiplying 3.90 x 20.2, and dividing by 26, I get the value 3.03. Looking in the chart, I find the closest match (using a small drive gear and tailhousing) is an 8-tooth tailshaft gear and a 24-tooth speedometer drive gear. If I had a large drive gear and tailhousing I have the choice of 12-tooth trans gear with a 37-tooth speedo drive gear or a 13-tooth trans gear with a 40-tooth speedo drive gear.

    Install the recommended gears, and check for correct odometer readings over a marked distance. Any errors will most likely come from poor estimation of the tire diameter, so you may want to measure it. If the odometer readings are correct but he speedometer readings are not, the problem is with the speedometer itself and it will need to be replaced.

    Good Luck and Happy Motoring!
    Source: Super Chevy Magazine
    October, 1989
    ________________________________________
    Speedometer Gear Colors
    Small (3/4") Gear
    Teeth Color
    17 Purple
    18 Brown
    19 White
    20 Blue
    21 Red
    22 Grey
    23 ???
    24 ???
    Large (1-1/2") Gear
    Teeth Color
    34 Light Green
    35 Pink
    36 White
    37 Red
    38 Blue
    39 Brown
    40 Black
    41 Yellow
    42 Green
    43 Purple
    44 Dark Grey
    45 Light Blue

    ________________________________________
    Speedometer Gear Chart
    Small Speedometer Gear
    (3/4" diameter)
    Trans Gear, Speedo Gear, Ratio.
    7 17 2.43
    7 18 2.57
    7 19 2.71
    7 20 2.86
    7 21 3.00
    7 22 3.14
    7 23 3.29
    7 24 3.43
    ________________________________________
    Trans Gear, Speedo Gear, Ratio.
    8 17 2.12
    8 18 2.25
    8 19 2.38
    8 20 2.50
    8 21 2.62
    8 22 2.75
    8 23 2.88
    8 24 3.00
    ________________________________________
    Trans Gear, Speedo Gear, Ratio.
    9 17 1.89
    9 18 2.00
    9 19 2.11
    9 20 2.22
    9 21 2.33
    9 22 2.44
    9 23 2.56
    9 24 2.67
    ________________________________________
    Trans Gear, Speedo Gear, Ratio.
    10 17 1.70
    10 18 1.80
    10 19 1.90
    10 20 2.00
    10 21 2.10
    10 22 2.20
    10 23 2.30
    10 24 2.40
    ________________________________________
    Trans Gear, Speedo Gear, Ratio.
    11 17 1.55
    11 18 1.64
    11 19 1.73
    11 20 1.82
    11 21 1.91
    11 22 2.00
    11 23 2.09
    11 24 2.18
    ________________________________________
    Trans Gear, Speedo Gear, Ratio.
    12 17 1.42
    12 18 1.50
    12 19 1.58
    12 20 1.67
    12 21 1.75
    12 22 1.83
    12 23 1.92
    12 24 2.00
    Trans Gear, Speedo Gear, Ratio.
    13 17 1.31
    13 18 1.38
    13 19 1.46
    13 20 1.54
    13 21 1.62
    13 22 1.69
    13 23 1.77
    13 24 1.85
    ________________________________________
    Trans Gear, Speedo Gear, Ratio.
    14 17 1.21
    14 18 1.29
    14 19 1.36
    14 20 1.43
    14 21 1.50
    14 22 1.57
    14 23 1.64
    14 24 1.71
    ________________________________________
    Trans Gear, Speedo Gear, Ratio.
    15 17 1.13
    15 18 1.20
    15 19 1.27
    15 20 1.33
    15 21 1.40
    15 22 1.47
    15 23 1.53
    15 24 1.60
    ________________________________________
    Trans Gear, Speedo Gear, Ratio.
    16 17 1.06
    16 18 1.12
    16 19 1.19
    16 20 1.25
    16 21 1.31
    16 22 1.38
    16 23 1.44
    16 24 1.50
    ________________________________________
    Trans Gear, Speedo Gear, Ratio.
    17 17 1.00
    17 18 1.06
    17 19 1.12
    17 20 1.18
    17 21 1.24
    17 22 1.29
    17 23 1.35
    17 24 1.41
    ________________________________________
    Trans Gear, Speedo Gear, Ratio.
    18 17 0.94
    18 18 1.00
    18 19 1.06
    18 20 1.11
    18 21 1.17
    18 22 1.22
    18 23 1.28
    18 24 1.33

    Large Speedometer Gear
    (1-1/2" diameter)
    Trans
    Trans Gear, Speedo Gear, Ratio.
    7 34 4.86
    7 35 5.00
    7 36 5.14
    7 37 5.29
    7 38 5.43
    7 39 5.57
    7 40 5.71
    7 41 5.86
    7 42 6.00
    7 43 6.14
    7 44 6.29
    7 45 6.43
    ________________________________________
    Trans Gear, Speedo Gear, Ratio.
    8 34 4.25
    8 35 4.38
    8 36 4.50
    8 37 4.62
    8 38 4.75
    8 39 4.88
    8 40 5.00
    8 41 5.12
    8 42 5.25
    8 43 5.38
    8 44 5.50
    8 45 5.62
    ________________________________________
    Trans Gear, Speedo Gear, Ratio.
    9 34 3.78
    9 35 3.89
    9 36 4.00
    9 37 4.11
    9 38 4.22
    9 39 4.33
    9 40 4.44
    9 41 4.56
    9 42 4.67
    9 43 4.78
    9 44 4.89
    9 45 5.00
    ________________________________________
    Trans Gear, Speedo Gear, Ratio.
    10 34 3.40
    10 35 3.50
    10 36 3.60
    10 37 3.70
    10 38 3.80
    10 39 3.90
    10 40 4.00
    10 41 4.10
    10 42 4.20
    10 43 4.30
    10 44 4.40
    10 45 4.50
    ________________________________________
    11 34 3.09
    11 35 3.18
    11 36 3.27
    11 37 3.36
    11 38 3.45
    11 39 3.55
    11 40 3.64
    11 41 3.73
    11 42 3.82
    11 43 3.91
    11 44 4.00
    11 45 4.09
    ________________________________________
    Trans Gear, Speedo Gear, Ratio.
    12 34 2.83
    12 35 2.92
    12 36 3.00
    12 37 3.08
    12 38 3.17
    12 39 3.25
    12 40 3.33
    12 41 3.42
    12 42 3.50
    12 43 3.58
    12 44 3.67
    12 45 3.75
    Trans Gear, Speedo Gear, Ratio.
    13 34 2.62
    13 35 2.69
    13 36 2.77
    13 37 2.85
    13 38 2.92
    13 39 3.00
    13 40 3.08
    13 41 3.15
    13 42 3.23
    13 43 3.31
    13 44 3.38
    13 45 3.46
    ________________________________________
    14 34 2.43
    14 35 2.50
    14 36 2.57
    14 37 2.64
    14 38 2.71
    14 39 2.79
    14 40 2.86
    14 41 2.93
    14 42 3.00
    14 43 3.07
    14 44 3.14
    14 45 3.21
    ________________________________________
    15 34 2.27
    15 35 2.33
    15 36 2.40
    15 37 2.47
    15 38 2.53
    15 39 2.60
    15 40 2.67
    15 41 2.73
    15 42 2.80
    15 43 2.87
    15 44 2.93
    15 45 3.00
    ________________________________________
    16 34 2.13
    16 35 2.19
    16 36 2.25
    16 37 2.31
    16 38 2.38
    16 39 2.44
    16 40 2.50
    16 41 2.56
    16 42 2.62
    16 43 2.69
    16 44 2.75
    16 45 2.81
    ________________________________________
    17 34 2.00
    17 35 2.06
    17 36 2.12
    17 37 2.18
    17 38 2.24
    17 39 2.29
    17 40 2.35
    17 41 2.41
    17 42 2.47
    17 43 2.53
    17 44 2.59
    17 45 2.65
    ________________________________________
    Trans Gear, Speedo Gear, Ratio.
    18 34 1.89
    18 35 1.94
    18 36 2.00
    18 37 2.06
    18 38 2.11
    18 39 2.17
    18 40 2.22
    18 41 2.28
    18 42 2.33
    18 43 2.39
    18 44 2.44
    18 45 2.50



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