Overdrive Speedometer Gears
for the Silver Ghost

 compiled by Gil Fuqua

Rear axle Gears Gear Ratio Approx RPMs at 60 MPH1 Tire Diameter2 MultiplierGears3 # Teeth4 Gear Color GM Part # 14/52 3.71:1 2,136 35” -- 36 White 1359270 15/52 3.47:1 1,998 35” -- 34 Lt. Green 9774413 16/52 3.25:1 1,871 35” 1.11 35 Pink 9780387 17/52 3.06:1 1,762 35” 1.25 37 Red 1359271 18/52 2.89:1 1,664 35” 1.25 35 Pink 9780387 19/52 2.74:1 1,577 35” 1.25 34 Lt. Green 9774413

¹Approximate RPMs based on 35” tire diameter in top gear.

 ²Tire diameter for a 33 x 5 tire on a late Ghost is 35”.  Note that a larger tire diameter will result in a lower speed reading.

 ³Multiplier gears part numbers:

             1.11 = GM 15635469

            1.25 = GM 15636219

 Note that multiplier gears are added externally at the overdrive. 

 ⁴The higher the number of teeth, the slower the speedometer reads.  If your tires are larger or smaller than 35” in diameter, you will have to fine tune the speedometer gear to match your actual speed.  To determine the exact gear, take actual RPMs at 60 MPH in top gear and multiply by 0.017.  This provides the correct number of teeth for the gear.  If the number is less than 34, you have to add one or more multipliers in the drive line to end up with a gear with teeth between 34 and 45, the range of gears that is available. 

 For example:  a 19/52 rear axle has 1,577 theoretical RPMs at 60mph.  Multiply 1,577 x 0.017 = 26.81. Since the result is lower than the available range of gears available (34-45 teeth), you have to multiply by either 1.11 or 1.25 (the speed multipliers available) to determine the result that is closest to a gear that is available in the range of 34 to 45 teeth. In this example, 26.81 x 1.11 = 29.76 and 26.81 x 1.25 = 33.51.  Since 33.51 is the closest result to an available gear, you would use a 34-tooth gear with a 1.25 multiplier.

Section 1 - Disconnecting Torque Tube
Section 2 - Removing the Front Section of the Torque Tube
Section 3 - The Mathematics of an Overdrive
Section 4 - Checking Universal Joint and Bearnings
Section 5 - Installing Overdrive in Drive Line
Section 6 - Overdrive Lubrication
Section 7 - Connecting Speedometer and Electric Controls
Section 8 - Hooking up the Electronic Control Box
Section 9 - Hooking up Speedometer and Calibrating Output
Section 10 - The Overdrive in Operation
Section 11 - Overdrive Service
Section 12 - Crusing in Overdrive
Endnotes - Endnote
Table 1 - Silver Ghost Rear Axle Gear Ratios Comparisons
Table 2 - Engine Revolutions per Tire Turn
Table 3 - Overdrive Speedometer Gears for the Silver Ghost