The Mathematics of an Overdrive, or How a 22% Overdrive Boosts Top Speed by 28%

 A conventional gearbox provides a straight-through output in top gear with a one-to-one relationship between engine revolutions and the output shaft of the transmission.  An overdrive provides a mechanical advantage with an output that is greater than one-to-one.  The Gear Vendors overdrive has an output of one revolution for each .778 input revolution from the transmission and is normally referred to as a 22% overdrive (1.000 - 0.778 = .222 or 22.2%).

 Simple logic would indicate that a 22% overdrive allows you to go 22% faster at the same engine speed – Not True!  Road speed is the reciprocal of the overdrive’s output (1.000 divided by .778 = 1.285).  The Gear Vendors 22% overdrive extends your cruising speed by 28% at the same RPMs compared with top gear without overdrive.  For example, if your Ghost’s optimum cruising speed in top gear is 60 mph, it will increase to approximately 77 mph with the Gear Vendors overdrive (60 mph x 1.285 = 77.1 mph) based on the same engine RPMs of 2,136.

 Tire revolution, which is proportional to ground speed, is affected by three factors: (1) the revolutions of the drive shaft, (2) the rear axle ratio, and (3) the diameter of the tires.  For example, a Silver Ghost with 35” diameter tires (a 33 x 5 tire standard on a late Ghost has a 35” tire diameter) travels 9.161’ per tire revolution (35” (tire diameter) divided by 12” = 2.917’ x pi (3.141) = 9.161’).  In one mile, the Ghost’s tires revolve 576.4 times (5,280’ (feet in a mile) divided by 9.161’ = 576.4 tire revolutions per mile) and it is also equal to the number of tire revolutions per minute at 60 mph.

 A Ghost with a 14/52 ring and pinion has a rear axle ratio of 3.71 (52 tooth ring gear divided by 14 tooth pinion gear = 3.71).  Traveling at 60 mph, engine RPMs are approximately 2,136 and can be calculated by multiplying the rear axle ratio of 3.71 times 576.4 revolutions per minute of the rear tires.  With the Gear Vendors overdrive engaged, engine RPMs will be approximately 1,662 at 60 mph (.778 x 2,136 (RPMs at 60 mph without overdrive) = 1,662), or 22% less.  Alternatively, RPMs in overdrive at 60 mph can be calculated by multiplying the effective rear end ratio in overdrive (2.89 for a 14/52 ring and pinion) times 576.4 RPMs of rear tires = 1,662.  In effect, engine RPMs at 60 mph in overdrive are the same as engine RPMs at 46.7 mph in normal high gear (.778 x 60 = 46.7) without overdrive.  See Table 2 for comparison of engine revolutions per tire rotation.

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