Written by: Mike Ervin

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.  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.
 

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
 

*   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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