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GM Speedometer Recalibration
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Ratio Order Disclaimer

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
17Purple
18Brown
19White
20Blue
21Red
22Grey
23???
24???
Large (1-1/2") Gear
Teeth Color
34Light Green
35Pink
36White
37Red
38Blue
39Brown
40Black
41Yellow
42Green
43Purple
44Dark Grey
45Light 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
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
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
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
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
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
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
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
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
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
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
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
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
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
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
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
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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Main Page Copyright © 1999 Scott Windle Created on: 09/08/99