Calibrate Your Speedometer For New Rear-end Gears For 700R4's

Written by: Mike Ervin

Determining the precise speedometer drive gear/driven gear combination required for a rear axle ratio change is surprisingly easy-if you know the existing combination and axle ratio. Whip out your calculator and follow along. Let's assume that your truck is currently equipped with 2.73 gears and that you're planning to install a 3.73 ring and pinion. If you didn't change speedometer gears, actual vehicle speed would 73.1 percent of indicated speed (2.73/3.73 = .731). That being the case, what's required is speedometer gearing that will result in the driven gear spinning at 73.1 percent of its current speed. The speedo gear combination for a 2.73 rear (assuming 29-inch diameter tires) is a 18-tooth drive gear and 35-tooth driven gear. Switching to a 45-tooth driven gear (the most teeth available) would translate to the vehicle traveling at 78 percent of indicated speed - close but not perfect.

Obviously, the drive gear must be changed to one with 15 teeth if the 73.1 percent ratio is to be achieved. Going back to the original 35-tooth driven gear for illustration purposes, the 15-tooth drive gear would result in the vehicle traveling at 83 percent of indicated speed - that seems like a step in the wrong direction, until the driven gear is changed. (This is where it gets a little tricky because you have to work with a percentage of a percentage). Matching the 15-tooth drive speedometer gear up with a 40-tooth driven gear brings the drive ratio to 72.8 percent (15/18=.83; 35/40=.88; .83 x .88 = .7304, which rounds to .73, or 73 percent).  There you go.

My truck came from the factory with 2.73 gears and 29-inch tires and as the above example shows it had a 18 tooth drive gear and a 35 tooth driven gear like the ones pictured here. Speedo Drive and Driven Gears I changed to 28-inch tires and I needed a 37 tooth driven gear to go with the original 18 tooth drive gear.  When I changed to 3.73 gears I needed a 15 tooth drive gear and a 41 tooth driven gear with the 28-inch tires.  Also I had to change the sleeve to #25512339 for 40 tooth up driven gears.

In case you haven't put all the pieces together, there is a distinct limitation regarding speedometer accuracy and steep rear gearing. Unless driven gears with more than 45 teeth, and/or drive gears with fewer than 15 teeth become available, it will be impossible to have an accurate speedometer with rearend gear ratios lower (higher numerically) than 3.90:1 - unless 28-inch or larger tires are installed.

Here is a site that has a  Speedometer Calibration Program  you can use to help calculate what gears you might need.
 


#Drive Gear Teeth X Axle Ratio X (20168/Tire Diameter in inches)
--------------------------------------------------------------------   = # Driven Gear Teeth
                                                   1001

Example:  15 teeth drive gear, 3.73 axle ratio, 28" tire diameter.

15 x 3.73 x (20168/28) = 40300

40300/1001 = 40.26 or 41 teeth
 

Here is a list of GM part numbers for drive and driven speedometer gears, as well as some combinations that are known to work:
 

Driven Gear and Sleeve PN's
 

Part No.
9774413
9780387
1359270
1359271
1359272
1359273
1362048
1362195
1362049
1362196
9780470
9775187
Teeth
34
35
36
37
38
39
40
41
42
43
44
45
Color
Lt. Green
Orange
White
Red
Blue
Brown
Black
Yellow
Green
Purple
Dk. Gray
Lt. Blue
Sleeve
25512340*
25512340*
25512340*
25512340*
25512340*
25512340*
25512339**
25512339**
25512339**
25512339**
25512339**
25512339**

*   Sleeve #25512340 for 34 - 39 teeth gears
** Sleeve #25512339 for 40 - 45 teeth gears

Blue Drive Gear Combinations

With drive gear number 8640518, (blue, 18 teeth) the following combinations apply:  (The 18 tooth gear I had in my 85 C10 was green, they changed the color to blue)

Axle       Tire Dia.      Driven Gear
Ratio    (Nominal)   (Tooth Count)
2.73        29.0        35
2.73        28.0        37
2.73        27.0        38
3.08        29.0        39
3.08        28.0        41
3.08        27.0        42
3.23        29.0        41
3.23        28.0        43
3.23        27.0        44
3.42        29.0        44
3.42        28.0        45

Red Drive Gear Combinations

With drive gear number 8640517, (red, 17 teeth) the following combinations apply:

Axle       Tire Dia.      Driven Gear
Ratio    (Nominal)   (Tooth Count)
3.08       29.0          37
3.08       28.0          39
3.08       27.0          40
3.23       29.0          39
3.23       28.0          41
3.23       27.0          42
3.42       29.0          41
3.42       28.0          43
3.42       27.0          44
3.73       29.0          45

Gray Drive Gear Combinations

With drive gear number 8642620, (gray, 15 teeth) the following combinations apply:

Axle       Tire Dia.      Driven Gear
Ratio    (Nominal)   (Tooth Count)
3.08       27.0           35
3.23       29.0           35
3.23       28.0           36
3.23       27.0           37
3.42       29.0           37
3.42       28.0           38
3.42       27.0           39
3.73       29.0           40
3.73       28.0           41
3.73       27.0           43
4.10       29.0           44
4.10       28.0           45

Lower ratio (higher numerically) rear or smaller diameter tire requires higher driven gear tooth count. Increasing or decreasing driven gear tooth count by one will usually accommodate a 1/2" to 1" change in tire diameter.
 

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